Probably the easiest way is to use a pass through encrypter like the Addonics CipherUSB. That's what I use, for reasons that seem to match most of your criteria: (1) It's easy. (2) It works. You can order them for around $30 bucks.
"Carrying your personal data on a flash drive is convenient, but it is not especially secure. If you were to lose the flash drive, all your data would be easily accessible to others. Encrypting your drive protects against such a scenario by making your data irrecoverable without the appropriate pass phrase. If you have Windows 7 Premium or Ultimate editions, you already have the capability to encrypt your flash drive. If you have a lessor version of Windows 7, then there is no point in upgrading. Several free drive encryption programs are available online."