This is surprisingly easy to do.
1) Purchase and download Lion from the Snow Leopard App Store.
2) Run it once its finished (should open automatically)
3) Click continue, and then agree to the license terms (don’t forget to spend the next 3days reading them first)
4) Your now on the important screen, by default your internal drive that has SnowLeopard will be selected as the install destionation.
BUT you should also see a “Show all disks” button. Click on this and then select your USB drive. Read More…