It is all in the sticky at the top of the page, but if you want the "Janet and John" version:-
- Fill out a CAA form, pay them £15 for reading it.
- Fill out an FAA form, naming an FAA FSDO where you want to get your licence
- Wait until told (takes a few weeks)
- Go to FSDO with JAR licence, logbook
- Receive temporary licence.
- Do FAA BFR
- Fly on temporary licence until your shiny plastic one, with a picture of Wilbur and Orville on the back arrives in the post.
That's it, no more than an hour or two with an FAA instructor, no FAA medical, hardly any money. However, you can't do it all in the UK, since there aren't any FAA FSDOs here.
G