Quick fitting instructions, don't be scared: it takes more to read than to do.
UPDATE, I'VE MADE A INSTALLATION VIDEO
https://www.youtube.com/watch?v=HGHWK_vgTjw&t=3s
1) disconnect the battery of your car and wait some minutes. Make your wheels straight first.
2) remove the steering wheel. Many tutorial are available on youtube, but it's very easy:
2.1 - unscrew the 2 bolts that hold the airbag in place, they're in the back side of the steering wheel.
2.2 - unplug the airbag connector, I use a flat screwdriver. A clip may be present to unlock the connector.
2.3 - unplug horn connectors in the steering wheel internal.
2.4 - uncrew the big nut in the middle, WITHOUT PULLING IT OUT YET.
2.5 - WITH THE NUT UNCREWED BUT STILL IN PLACE, pull the steering wheel until it unlocks from the spline. After years, the metal coupling between steering column and steering wheel can make a kind of glue effect, so you better unlock this coupling pulling towards you WITH THE NUT STILL IN PLACE, because the steering wheel can unlock suddenly and you can hit your face and tear the wires.
2.6 - when the steering wheel is unlocked, screw out the nut and pull out the steering wheel too.
3) fit the wire extension included in the kit, for the horn wire
4) fit the female-splined hub in the steering column and fix it tightening the nut that was holding the steering wheel. I've made spline a bit tight, so you may need to push it in with a rubber hammer. When you see the thread of the column coming out, you can make the female splined hub slide in by tightening the nut.
5) Fit the black (or other colour) cover on the female splined hub, and the male splined hub inside it too. The black cover is hold in place by tightening the male/female splined hubs each other, using the 4 bolts. Use loctite or other threadlock. Make the wires cross the slot inside the flat hedge in the middle of the black cover.
6) Fit the steering wheel on the male splined hub, keep care that it must be straight because Porsche didn't make any extra tooth or reference avoiding wrong angle while refitting the steering wheel. Also, as mentionned in point 4, the spline can be tight and you must hit the internal of the steering wheel with a rubber hummer until you see the thread of the hub comng out from the center hole of the steering wheel. At that point, fit the washer and the nut provided and tighten it until the steering wheel slides all on the male splined hub.
7) Fit back the airbag in the steering wheel.
8) re-connect the battery.
9) enjoy.