Here's a quick way to do it that came off MTBR.
I've use this method myself and it's dead easy.
Let the air out of pos/neg chambers.
Remove both valve cores.
Compress fork and attatch a toe strap to hold fork compressed.
Uncsrew 10mm nut on bottom of left leg till it is flush with the end of the neg air chamber threads.
Tap the nut with a soft hammer to unseat the air chamber (or air piston rod) from the slider.
Some oil will drain out so have a catch pan ready.
Remove 24mm top cap from left leg.
Use a thin rod (I use a Park 5mm L handle allen) to carefully push the air piston out of the top of the left leg.
Carefully push the allen through the hole in the bottom of the slider against the air piston rod until the air piston comes out the top of the leg.
Under the air piston is the spacer if the fork is set up for 80mm, or no spacer =100mm.
If going from 80 to 100 remove spacer, 100 to 80 add spacer.
Carefully push air piston down past threads inside stanchion about an inch.
Add 3-5cc 5wt Rockshox oil on top of air piston.
Install positive air cap (24mm) and torque to 60 in/lb.
Install pos chamber vave core.
Turn fork upside down and with a syringe add 15cc of a 50/50 mix of 5wt and Red Rum through the hole in the slider.
Using a shock pump on the pos chamber add enough air to force the threaded portion of the piston rod out the bottom of the slider.
Install the 10mm nut and torque to 45 in/lb.
Remove the toe strap, inflate the pos chamber to desired psi, inlate neg chamber 10 psi less.
Install air cap covers, clean fork with isopropyl alchohol and you are done.
Sorry that was so long, I do at least 3-4 of these a week at the shop, takes me 10-15 minutes.
All torque values and oil specs are from my recent trip to SRAM Technical University.
Robb
