Step one is to remove the tailhousing.  Step two separates theintermediate shaft from the main case.  Here, all synchros, a couple of dogs and 1st, 2nd-3rd sliders were replaced.  Ouch.
 

In a 901, everything is attached ot the intermediate plate. The trick comes when it's time to assemble it, the shift forks need to be in the right place, and you have to hold the shafts in the right position when you drop it into the main case.
 

We use DowCorning 730 exclusively.  Good stuff, expensive, but it seals.

After adjusting 1st-Reverse slider pop on the tailshaft.

Last item on this particular 901, a side shifter, was to replace the shifter arm seal.  You need an internal bearing puller, but otherwise it's strightforward.

We also replaced the output flange seals, the input shaft seals, and the diff housing cover gasket/o-ring.