I gave a bit of a description of what bias does here. I'm reluctant to give a detailed how-to procedure because its very likely someone's amp would get fried if I made a mistake, or my instructions were misinterpreted.

Power tube bias adjustment is important enough that its worth having a professional tech do it for you. While they're setting it up, they may catch other potential problems before they become serious. Compared to the cost of the amp, the fee for bias adjustment is a good insurance policy.