It's pretty straightforward, you replace the entire knob/gaitor/plastic surround trip in one go. If I remember correctly, the gear lever needs to be in the Neutral position.
Lift up the plastic trim, which will expose a catch type mechanism which as I recall you need to pop up.
Then grasp the shaft firmly and give it a good yank. The entire assembly - knob, gaitor, and plastic trim - will come off. The button on the knob will now portrude out. Make sure that it's in the same position on the replacement unit!
Then, as all good Haynes manuals say, reassembly is the reverse of disassembly.
The end result:
Actually, I left a note on my flickr image page about the procedure
put in neutral, lift up the plastic, disconnect the cable, remove the retention bracket (bottom plastic piece so it slides up) and then give it a good yank upwards. The new one then just slots straight on.