Go to Preferences in iTunes, and go to Advanced and select the General tab. From there, with your USB drive mounted on the desktop, you can change the iTunes library location to there.
There are various ways to copy the music over. the way I did this was to delete all of the tunes from the iTunes library (but NOT send them to the trash when prompted) and then go to Preferences, Advanced and select Copy music to library when adding.
I then went to File > Add to Library and selected my library on the internal drive. Once the copying was completed (you'll want to check on your external drive that the files copied over!!!) I erased my internal drive's library.
Be aware that you must have the USB drive plugged in before launching iTunes, or else iTunes will revert to the original internal drive location until it is closed. This can lead to duplicated files and general file location confusion!
J