Evilkitty's info is correct, and
there's a step by step guide here, with links to the forms etc - scroll down a screen to get it.
There's no cost at the US end.
Incidentally, the FAA license you end up with isn't temporary - it lasts as long as your JAA licence remains valid - all you have to do extra is undertake the Biennial Flight Review that all FAA PPLs must do every two years anyway.
The 'temporary' confusion perhaps comes from the fact that the FSDO that you present your details at issues you there and then with a 'Temporary Airman's Certificate' (Lasts 90 days IIRC) that allows you to fly immediately i.e. before you get your credit card sized FAA license sent to your home address.