There are some good tutorials on this forum. The Pelican tutorial on 101Projects is good, but if you go ahead and take out the headlamp switch first and that vent you can do most of it working through that hole, and not have to crawl down on the floor peering up under the dash. There are a couple of good threads on here with pictures.
Steve
Edit: this is the thread that helped me:
http://986forum.com/forums/general-discussions/46411-ignition-switch-replacement-holy-crap.html
My contribution to the thread is on page 2, the screwdriver set that let me do everything through the hole where the headlamp switch/vent was. The diagram on page 1 shows more details too.