How to set custom white balance?

What is the best way to do this? I'm using a Minolta 7D.

Can I use a sheet of paper? Or do I need a white card?

Any other tips on doing this?

Best bet is to check the manual. Most cameras work best with a white sheet of paper (like the stuff you would print a letter on, nothing special) but I've heard of some people using a grey card as well.

Another option is the Expo Disc which screws on the front of the lens like a filter. They you shoot the scene (setting custom WB) with this thing on the lens and you're all set. They're fairly expensive and I've also done something similar to that method with the translucent plastic cover from a can of Pringles. Works great and cheap.