my steering rack leaked, so I had to take the car into the shop for a repair.
I watched the mechanic perform the complete action in 4 hours (not bad eh...).

What you need is a 928 steering rack repair kit (a bunch of rings and seals, orig. 928 part) 928intl have them.
Normal socket wrench set, and the normal tools A hammer and driving tools, Patience, and preferably a shop-lift or a decent garage pit.

This is a global step by step:

1. Take off the belly cover plates (both the metal and the fiber one)
2. Start wiping off all the dirt so that you prevent dirt getting into the rack and damage it again.
3. Push down the boots on both sides to the far wheel end, and let fluid come out.
4. Unscrew the bolts that attach the fluid lines from the pump to the rack, and let all the fluid drip out
5. Unscrew the tie rod nuts (and lock-nut) on both sides and screw out the rod's
6. Unscrew all smaller nuts and loosen the bigger nuts to provide some maneuverability
7. Unscrew the bolt from the steering axle, and carefully push the splined shaft out (hammer req?)
8. Remove the steering rack completely. Check for the condition of the mounting rubbers. This is a good time to fix them too.
9. Take off the metal fluid lines from the rack (really requires force on the bolts, had to use a hammer/driver)
10. Remove the cover plate and the rack guide on the side, and take out the spring and guide-block
11. Remove the 3 bolts that hold the top plate, the plate, and the bearings with some rings. Mind the sequence.
12. Remove the top axle with the steering worm gear gently by rotating a bit.
13. Remove the two aluminum stopper rings from the main shaft ends
14. Use a driver and hammer to loosen the 2 ring-screws from the yellow 'main-pipe' part to both aluminum ends
15. Carefully take apart the main housing now, thus ending up with the right alu end, the 'pipe' housing, the left (steering) alu part.
16. Gently take out the axle from the housing. Take off the inner guide housing and the rings and c-clips.
17. Now start replacing all rings, and seals that you have seen. Use the sockets from your wrench set to drive out the seals gently, and make sure not to damage the housing.
18. Put it all together again in reverse order. Carefully align the aluminum housing parts again.
19. Reassemble the worm gear. Take good care not to damage the worm bearings when re-assembling. Use some grease to keep the bearings seated properly if required. Make sure the parts move smoothly.
20. Remount the metal fluid lines, and use new O-rings as supplied with the set.
21. Mount the rack under the car, and connect the steering axle to the splined shaft. Bolt on tight (clamp).
22. Re-attach the fluid lines and reconnect the tie-rods again. Test for proper steering movement.
23. Move the boots back in place and make sure they are properly seated. Dirt must be kept out.
24. Mount the cover plates again, and top up the reservoir with ATF fluid (Dexron II / III)
25. Start the car, and let it idle for a moment. Fill up the reservoir again. The rack will fill itself with ATF.
26. Gently start turning the steering wheel and let the air out of the system. The system will make a humming sound.
27. Again top-up the reservoir and do a drive around the block, testing the steering for proper alignment
28. When the sounds start to disappear, make some complete turns to left and right. All ok??

29. Now go to a decent alignment shop and have the wheels aligned. 
30. Clean all the ATF you spilled all-over, and have a beer.

It is not a wise idea to start racing after reassembling the steering rack.
Test it for some time before relying on it.
Check the fluid level in the reservoir every other day. Also watch the cover plates for ATF oil dripping.
Wheel re-aligning will be necessary whenever you had the tie-rods loose. 

This is not my personal achievement... but I think in future I would take on the job. It is not so hard.
A good thing to do in such a job, is to organize all parts in the right sequence, and make some notes/drawings.

Makes reassembling all these bits and pieces easier I guess. It is annoying when you have to take it all apart again when you find out that suddenly you car has a personal favor for hard left turns....

I have an exploded view diagram of the Zf rack that I can email upon request.

'88 928s4 cherry red
The Netherlands