The first step you need to take is to remove the uppers from the lowers (lower legs from the sliders). To do this you need to undo the Allen key bolts, 1 at the bottom of each leg. Before you remove them completely though, leave them slightly threaded and give them a tap with a hammer or something to free them up a bit as they often get stuck. I think there is also a reducer washer to keep an eye out for in the lower leg. You need this for the air carts, but not the standard judy ones.
Next up is getting out the air cartridges. You should be able to simply undo them from the top/crown. From memory they are plastic so go carefully. Once you have undone the bolts the air cart upgrade should slide out from within the slider. You may need to undo the bolts that hold the crown to the slider if you have that era of fork, as they often prevent the unscrewing of the top caps.
This is the end of the air cart part. Don't listen to fluffy, you need to pop them in the post to me
If you don't have the original innards, I wish you luck in your search, but if you do, as fluffy has indicated, you need circlips/circlip pliers, a damper for one leg, a dummy damper of the same travel for the other, then atop these sit the springs and/or elastomers depending on which you have.
Assembly should be to fit the damper/dummy damper in the sliders from the bottom, then fit the retaining washer and then the circlip. Same action both sides.
You can now fit the springs into the sliders, fit the top caps, then put the sliders back in the lowers and do the bolts up (not too tight, you can crack the lowers). The bolts from the air carts will be too small for original judy internals so you will need to replace these too, unless you already have them.