How does one learn the correct way to install a ring and pinion gear set into a 901 gearbox? What tools/fixtures are required to do this, and where can they be found?

I have successfully been into transmissions several times for my race car, and this was made possible by resources from Dr. Evil; Thanks a bunch Mike!

Now I would like to be able to install another gear set or LSD module.

The best place to look is the factory repair manual, it has the procedure and the list of tools needed, but there are errors, mostly omissions.
Tool that is most important is the VW 385 and complimenting attachments.
I installed a factory ZF LSD and it went smoothly.

Another possibility to measure pinion depth (other than the VW385) is the P258. This was the tool that Porsche supplied in 1965 when they introduced the 911. They modified it in 1969 when they changed the transmission cases from aluminum to magnesium. Look at an ealy service manual to see how it works. A P258 is very hard to find but I bought one over Samba a couple of years ago, had the bushings made for the mag case and the the setting block modified for the mag case. Sold it for about $700 which is half of what a VW385 costs.
Pictures attached.
The kicker is that the VW289d tool uses the same casting as the P258 so it is possible to machine the VW289d to make a P258. I've had serveral made (sold for about $700) but now my machinist refuses to make any more because the tool is case hardened and very hard to work. Also VW289s are hard to find now. I have pictures somewhere but can't find them now but if you look at Pelican 911 forum, a guy has a good thread on rebuilding a 915 transmission using the tool. Name was evan911, I think.
I am now working on modifying a VW382 but have to see how it turns out.
Lastly, other people have made P258 type tools by modifying differentials to carry a dial indicator. Look around in the internet.Click to view attachment Click to view attachment

P258 Reproduction.Click to view attachment