Basically Get your wheel (make them out of 18mm plywood it works fine really!) and drill an 8.5mm hole in the middle. Then lock the drill so it cant turn and then you can simply screw it on and this will tap the wheel.

Nice 100mm diameter wheels are available from b&q and seem fine although not huge grip.

As for mounting them there are manyways and from my experence the way dom mods them (not mounts them as that is fine) is not the greatest. Better off putting set screws in instead of leaving the ball bearings.

Best way and simpelest I have seen yet is that used by rip. However I will not say how to do this as I am not sure if Jonno and Steve want everyone to know there way.

Regards
Ian