I did my gear knob modification the other day and I decided to post some photos for the potential weekend DIY
I shortened my gearstick ~4cm from the top.
1. Remove the OEM gear knob. The best way I found was to leave your car out in the sun for about an hour and approach the pulling and twisting that everyone's talking about from the passenger seat.
2. Use a towel or some newspapers around the gear stick and some tape. This will prevent metal particles landing on to your stereo, seats, handbrake etc
3. Use a hacksaw to cut the metal rod to your desired length. I cut mine about 4cm.
4. I used some electrical tape around the tapered end of the rod to make it more uniform and parallel to the rest of the rod. This step isn't really necessary, but I thought this would stabilize the gear knob better. Refer to the photos below.
5. Just follow the standard gear knob installation. Insert the rubber cap at the tip of the gear stick and sit the new knob in place. Use the allen key to stabilize the 3 holding screws.
This is my rough guideline. If you've done it in some other way, perhaps you could recommend your own procedure on this thread.