How do you use a tattoo gun?

Well, first off, I think you have to be formally trained apprenticed. I really wouldn't advise it if you don't know what you're doing. At the least you're going to get something ugly, at the most, it'll hurt like heck.

If you'd like to know how a Tattoo gun works, however, this site is useful.

Tattoo Guns - LoveToKnow Tattoos