Here's the tarnslation... I take NO credit for this, I'm just translating it...
First, cover the paint of your Z with a cloth or something similar
Find the cover of the seat belt roller and remove it. It's held in place by pressure, and can be removed with a simple move. [looks like it needs to be pulled up and backward]
By moving the seat belt you'll gain access to a Torx screw: unscrew it and and remove the plastic seat belt guide.
With the help of some wood wedge (do not use a screw driver because it's very easy to ruin the plastic cover [if you check with Harbor Freight Tools, BavAuto or auto supply store they have nylon coated tools used to remove door panels and such]) try to open the latches holding the roll bar covers
A detail of the latch...
At the first try with the right cover, I broke two latches, but it's not a big deal...
Once you remove the rear cover, we can use a screw driver as a lever to remove the front cover. Use these holes...
The rollbar will look like this:
At this point, drill the plastic covering the rollbar to place the guides by the holes already on the rollbar. Before closing up everything remember to place the inner part of the wind breaker...
Once you're done, close everything up!
[if you buy the BMW wind braker, ask for the installation instructions: parts must give them to you]