It's not too difficult, You just have to take the time to think out all of the motions that you need. A simple biped, like the one that squirrel posted, only needs two different movements: One to shift the weight from one leg to the other, and one to move the legs forward/backward. If you design your bot right, you should only need two motors/servos.
I made a pretty crummy one a while a go, but it works. One motor to move the legs forward and backward, and one servo on each leg for an ankle.
http://www.chiefdelphi.com/media/photos/25362
Good luck with yours