If you pop off the throttle body, you'll notice where it's partially shrouded. You line the outside of the throttle body where the shroud is with JB Weld or some other similar epoxy. Then you grind away that shroud. The JB Weld fills in the hole that you'd otherwise have by grinding that shroud away. I also took the time to (mostly) fill the idle air control hole, which got rid of the stupid throttle hang that the Zetec otherwise has. Don't completely fill it, because that's how your engine breathes at idle.
Do some searching around and you'll find plenty of posts about it. Buy a spare throttle body to do this on - that way you're not hosed if you screw it up. They're stupid cheap to pick up.