One has couple of options here.
Fill the rezzy up and tap out bubbles which would be ok if no adjusters on the rezzys. The issue you face is forcing air out between the IFP and the fluid as you install the IFP. With no way to control the flow back into the shock body as you insert the IFP, you run a risk of leaving some air trapped in between.
One can at that point Change the orientation of the rezzy and bleed back to the body cycling a few times to make sure it has been removed from the rezzy and line.

Since I have adjusters which can hold air trapped and I want to start with no additional air to bleed I opt for another method which helps with bleed time between the the two tubes.

While discussing the rezzy. Not all have an IFP and may have a bladder. This will have a slight change on the way you do you air bleed for the rezzy. First step is still first step. The rest will be covered in another post on works or go to a video some one here done and see what he did. What's important is no air left behind to cause issues.

The next step for me is fill the rezzy with fluid and have a way to control the bleed back pressure into the shock body as I install the IFP as well bleed air from in-between the IFP and fluid in the rezzy.
I use a home made tool to do this because I'm cheap. It acts like a ram inside the shock body. They do sell them. If you make one it has to be of a material that will not scratch the inside of the shock body tube.
I fill the body up to the thread with fluid then insert the ram and slowly push fluid into the rezzy and slowly pull on the ram to move fluid back and forth between the tubes. I turn the adjusters stiffer a few turned each time on compression and turn back to zero for rebound(return of fluid back to shock body). I do this and check for any air bubbles in rezzy. See pic. If you look close at bottom right of rezzy you will see an air bubble that has been set free from bleed procedure. Keep in mind this simply a pre bleed prior to IFP install. This helps later in final bleed after IFP has been installed.

Once I feel I've done what I can getting rid of air I the push fluid back into the rezzy about 3/4 full. I install o-ring and band on IFP. I then tilt the rezzy tube a little forward to create and slight angle for IFP to be installed with bleed hole in an upward position. I wipe down the o-ring and band with a little oil and insert the IFP so the o-ring just touches the rezzy body and bleed hole up. I set the adjusters to full open then hold the IFP with one hand and slowly push on the ram forcing fluid to the top of the IFP and out the bleed hole. And once I get a fluid flow I put pressure on IFP pushing the IFP into the body holding the ram. Once the o-ring seats I let pressure off both.
Now that I have a seal in the rezzy I pull up slowly on ram and watch the IFP move deeper into rezzy untill it bottoms out.
You can see in the first pic the fluid from the IFP bleed as it is being inserted and flowing down the out side of the tube. In the second you can see where the ram is up and IFP I'd bottomed out.

It may or may not complicated however it really is not one just need to play around with it then when comfortable do it with new parts and oil. Find what works best for you and than share with us of course.

Now that I have the rezzy first bleed and IFP installed I move on to final bleed of rezzy. I remove the ram from the shock body tube and install end cap on rezzy and use my hand brake bleeder to simply move the IFP up and down the rezzy tube. This way I can lower the rezzy below the shock body after I do a few bleed runs with it still in the up position cycling the IFP and moving adjusters removing any remaining air for the rezzy, adjusters and line.

Few pics of final bleed you can see the remaining air bubbles that showed up in the shock body tube.
Now that I feel good about rezzy bleed I can get ready to work on piston

Time for a little commentary. I mentioned a few time "slowly" do this this to keep from air bubbles from forrming and having to do it all over and over again as well as fluid spillage control. The fluid while filling /bleeding is at atmospheric pressure so air bubbles can form easy. The ram did have some air trapped between it and the fluid I was carefully not to bottom out the ram as well made sure there was enough fluid in the shock body. The ram I made allows for installing it in a slight angle as to bleed most of the air out and then set vertical for push and pull.

I mentioned I set my IFP for this shock at bottom for next step. This is important so when I remove hand vac and install valve core and pressure to hold in place I don't get a face full of fluid and have to do the whole thing over. The pressure in rezzy now can now be set with just plain air for shaft install it don't take much 20 /30 psi or so. I use 100 in this because that is just what I do. Plus this way I can remove end cap clean inside of rezzy and lightly grease the Back side of the IFP on final. This will lube the o- ring as the IFP is compressed in the compression stroke and not allowing for contamination if installed pre lubed.

Upon the compression stroke the fluid in the shock body is displaced via volume of exposed shaft entering the shock body forceing fluid intothe rezzy moving the IFP towards the end cap.

Different brand,typs etc each have a different IFP setting or depth with in the rezzy body or shock of no rezzy and has an IFP.
So if your shock was not leaking make a mental or written note of this. If your shock was leaking check other one or ask for depth number for your shock from MFG.

One can easily calculate the volume of the shaft in cubic in and change it to ounces and set it that way.

Do you have an ifp tool? I made one with a long 1/4x20 threaded rod with a handle on the end. Worked just fine. I was able to mostly gravity fill the ressy with the IFP in place. Pushed it back and forth. Once all air bubbles came out the shock body I then set the IFP to the factory distance and then filled the shock body with oil. Once filled to correct level, I pulled the IFP to make a lot of room for the main shaft/piston/valves. Enough room to slide that in/out several times without overflowing. Then raise the shaft to the top of the oil, set the IFP to fill height once again; lifting the shaft every time oil got close to overflow. Keep lifting and setting the IFP until at fill position. No oil overflowed. Snapring, then Cap the shock. 1 teaspoon of shock oil on top of the IFP for seal lubrication. Cap the ressy. Cycle a few more times and let gravity displace any left over tiny air bits if any. Then purge fill with nitrogen.

I’m sure and little bit of air left just ends up against the Underside of the IFP. Shouldn’t be much if any. The nitrogen pressure is gonna keep that little bubble in check anyways.

Great tips zero, thanks for sharing.
Zero shock is a fox 2.0 I do believe he also has no adjusters on the rezzy. Much easier bleed. To get out the tiny bubble he most likely lowered the rezzy below the shock body inlet hose and then held the rezzy so the air bubble will rise taping the can and pump it right out by cycling the IFP "slowly".
As far as a IFP tool goes, sorta. Me being a back yard hacker(name given by board member years ago and it stuck) I simply use a bolt and nut with a nut runner and some electrical tape. I put the nut on the bolt and run it down to get more surface area inside the nut runner. Longer rezzy body's need longer shaft. For my axis front this works well. I mainly use it for IFP removal. The cans are pretty short. I went out and shot a pic of the ram,IFP tool and fluid syringe I use for these.

Now that I have the rezzy done I work on the shock body. I need to replace wore out bottom out bumpers and replaced the seals while I was at it as well as do a fluid change.
In the first pic you will see the valve and shims on a bolt with a nut. What I do so I don't change drop loose etc. I take a bolt and hold it on the end of the shock shaft and slid the whole assembly onto the bolt and put a nut on it. I already have valve spec so no need to take apart and log. Also it makes it easy to clean the assembly pre lube and reinstall when ready. No lost parts or change by accident or put on backward etc. Simply remove nut and slide on when ready.

Now I can assemble the shaft. I install the seal and o-ring paying attention to the way they came out of the seal head. Be sure to pre lube the seals o- ring etc. Then I put back on the way it came off with all the parts. Be careful installing the seal head or bearing not to damage new seals. I recommend a shaft guide if not go slow and wiggle on. If it don't go don't force it. Older seals not hard. New seals more patience required.
Int pic of the shaft assembly I have the bottom out bumper, seal head and shim valve assembly. Then I put in vise and torque nut to 30 lb ft. That is the spec for my axis shock.
In the pic you will see the little round Allen head valve full open on the rebound just above the lower eye shock bolt location. You will also see the shaft bleed hole by the valve assembly.

Zero , Kuma and others can comment on how the fox is a little different assembly. Axis has no snap ring. Spin in and tighten up. One piece assembly. There is a bleed hole on the shaft head to bleed as installed.sorry I did not get it turned right for the pic I'll see if if I have an old one or add location to a pic.

Went on line and got a pic of the seal head that showed the bleed hole. The hole is in the side below the threads. The inners seal has groves that one lines up with this hole.

Once the shaft is all assembled and ready to go, put air in the rezzy to hold IFP bottomed out, check fluid level in shock body so I can install the shaft and start bleed. The final fluid level and IFP set point is not done as I need to bleed shaft and valve assembly. The two pics will show about how much displacement there is once you install valve assembly. Look at the elevation of fluid based on threads inside the body
Also note the air bubbles. I get rid the one air bubble before I install shaft. Then another pops up before I even cycle the shaft. Get rid of both then proceed. I start moving shaft up and down slowly beeding air from the valve and shaft(rebound) being careful not to raise the shaft too far as to expose the bleed hole.
The oil level remains due to air in rezzy not allowing the IFP to move. As the shaft goes deeper the fluid level rises. Now you see why you don't top off fluid prior to bleed. It will simply spill out. Tapping on the body gently and the end of the shock shaft by the rebound adjust screw helps air get out. Since I do have a rebound screw I need to move five to ten turns as I move shaft then return to full open.
This may several strokes to do and some sit time between. If not you will have air trapped to cause issues later.

Once I'm happy with the process. I raise the shaft to the top of the fluid level being careful not to expose bleed hole in shaft by valve. This allows the shock body to be toped out and fill to base / middle of the threads inside the body. Then I slid down the seal head paying attention to the location of the bleed hole. I tilt the shock a little as to help force out air.
Next I let the air out of the rezzy and remove valve core. This will allow some of the fluid to travel into the rezzy as I screw in seal head. Even with a zero IFP you need a little so the IFP don't slame the rezzy during rebound. Since I have compression adjusters they are set full open. I do not want all the excess fluid to poor out the shock body and allow some to get In The rezzy.
Now I start to push down the seal head into the body air and then fluid starts to come from bleed hole. Once fluid is flowing I seat seal head so it threads then turn down quickly to o-ring seat then Cargill watch o- ring start to seat and finish off.

This leaves me with no air or very minute air in the cylinder ( shock body/ rezzy) and I can check for cavitation. At this time I go ahead and pull rezzy end cap and check the IFP. I stroke the shock slowly feeling for vacation then speed up shaft and do the same. I watch the IFP just to make sure all is working correctly and IFP is not at full bump. Then I do the same working compressions and then rebound adjusters. If all is good I then reinstall rezzy end cap and valve core. Move shaft to middle position. Then I put in 100 psi and the shaft should top out on its own. If that passes I stroke the shock a few times adding comp damping and. Feeling for stiffness using different pressure by hand on shaft. If that feels good then I open compression and watch rebound speed and then add rebound speed.
Now I let out air from rezzy Lok inside to be sure all is good. Clean inside and out thin amount of grease at back of IFP cycle the shaft top to bottom wipe inside of body clean, reinstall cap valve core and charge to 200 psi. That is what mine takes for my shock set up.

On the subject cavitation I'll share a pic that simulates this. This is caused from a few different things,leaking seal,too low of nitrogen, leaking IFP. A shock in good shape has to have enough pressure against the fluid during compression and rebound to maintain a pressure balance so air bubbles do not form on the low pressure side. See pic for demo pic.


As I mentioned "slowly" several times. In this case for pic I use fast motion allowing for a high and low pressure zone and form air bubbles from cavation.
I strokes the shock quickly as not spil out fluid with IFP under 100 psi. This creates a high pressure zone on the compression side and the back side of the zone is atmospheric. This allows for the onset of cavation. See pic.
This can also happen with a good shock with a low nitrogen charge and a balanced amount of fluid to nitrogen pressure at high shaft speed.

Now that we can see an effect of shaft speed and pressure zones you can see how if we reduce psi to soften up compression it may have a valving issue on a high shaft speed event and lose all valving control. Another thing to consider with psi is reduced is the IFP travel speed and distance from lack of fluid pressure to force through the valve and shim stack allowing cavation on the back side.

As the shaft moves into the shock body fluid is forced through through the valving due to pressure applied. If no or low pressure acting on the IFP will allow the shaft to move quicker with out the valving doing its job. The amount of bleed also has an impact. See zeros post.

On IFP setting think about this and if a rezzy that was 12" long with not fluid in it to start and low pressure acting on it. You can see how the IFP will quickly move towards the end cap and fill the rezzy with fluid not leaving enough on the shock under pressure to flow through valving and just move thru bleed. It will vacate and most likey cause some sort of damage.

Final note. In order for your IFP to move it has to have more pressure acting on it to over come the static pressure set. The applied pressure is important to make sure the fluid stays stable as the valving moves through it.

Adnoh, aka back yard hacker

So what have you decided to do with your buggy? If covid don't screw it up on planning a ride to LS in september 7-11.So stix is only one in so far besides me,we just came up with plan today,you should come out

Thanks for the invite.