I did a few searches, but was left unsatisfied, so I documented my install this evening, hopefully it will be helpful to somebody. It is a fairly easy install, took me about an hour to do with one hand. Your results may vary.
First the tools I used. 27mm socket with short extension and a swivel on a big torque wrench. Phillips screw driver. 3mm hex T-handle. Dykes, wire stripper/crimper, electrical tape, crimp eyelet. Optionally you can use a soldering iron, shrink tubing.
Step One. Pull the Horn Relay (your neighbors will thank you). Here it is on my 85 Carrera, yours may look different, consult the owner's manual. Before you go further, Make Sure your wheels are pointed straight ahead.
Step Two. Remove the horn pad by pulling up on either corner. Wiggle gently, it should pop off with minimal force.
You should see this underneath, the 27mm nut that holds the wheel on.
Step Three. Using your torque wrench, breaker bar or socket wrench with extension, gently apply pressure until nut releases. As I did this alone, I used the steering wheel lock, and did it very slowly to make sure I didn't break the lock. You can have a friend hold the wheel in place, or brace it with something. The nut released fairly easily, and I unscrewed it the rest of the way.
Step Four. Using the Phillips screwdriver, remove the turn-signal cancelling ring from the back of the stock steering wheel.
Step Five. The existing Porsche Horn wire has a small crimp tab on it, Momo uses a larger one, and provides you with it. You will need to cut off the Porsche one and solder on the Momo. Alternately you can snip off just the crimp and attach a larger crimp eyelet, this is what I chose to do.
Step Six. Using the three Momo supplied screws, attach the turn-signal cancelling ring to the back of the Momo hub. The horn wire will go through the hole (you'll see).
Step Seven. Put the Momo hub on the spline, but do not push it down all the way. Once its on, its not easy to pull off, so for now leave about a half inch gap between the hub and the column.
Step Eight. Once you're sure everything has gone well with wiring the horn button, and you've mocked up the wheel and it looks straight, go ahead and tighen the 27mm nut to 50ft lbs on the old torque wrench. Attach the wheel to the hub with the six supplied screws using the 3mm hex wrench. Take it for a test drive, if the wheel is straight and the horn works (don't forget to reinstall the horn relay), go back and put some Green Loctite Threadlocker on those six screws.
Please let me know if I've forgotten anything and I will amend the post.