Ok - So having just been through most of this, here goes...
1) Start by downloading the correct CAA forms (there are two you will need; a payment form and Verification of Authenticity for Foreign Licence). They can be found here:
http://www.caa.co.uk/application.asp...detail&id=2090
and
http://www.pprune.org/forums/showthread.php?t=292482
Next, go to the FAA web site and search for an equivalent to the verification form (its easy to find - sorry I don't have the link).
The CAA will sting you for approx £40 and the FAA don't charge. The CAA will take about 6+ weeks to and the FAA approx 8+ to talk to each other. You will eventually be sent a form from the FAA explaining that your details have been confirmed.
2) find the location of the closest FDSO office in the part of the US you are staying in and arrange an appointment. You will need to have swatted up on your air law and RT (both are quite different in the US as I have recently discovered!). Infact in hindsight I would download a copy of their RT Phraseology if there is such a thing on the FAA web site.
3) turn up at FDSO office in US and get grilled for 2hrs. If they are still happy, they will grant you a piece of paperwork (NOTE - some people have also required a US medical.. you takes yer chance)
4) Arrange a flight test with an approved training school
The previous thread is here:
http://www.pprune.org/forums/showthread.php?t=292482
Cheers
Steve