First, if you do a search, you should find some good posts about it.
Second, you want to have some of these supplies:
-A drill press or a dremel with sand paper grinders
-Some kind of High Temp Epoxy
-Lots of patience
Start with building up the back wall of your tb with the epoxy, I would use several cotes of the epoxy to be sure it is thick and strong enough.
For Saftey's sake, Let it dry overnight!
Then use your dremel or drill press w/some good grit sandpaper and gently shave off the restrictor/lip that hangs over the throttle opening.
That's pretty much it, just be sure to clean it before re-installing, you don't want plastic shavings in your engine.