If trying to make your own gaskets with a roll of gasket paper you can buy:

Lay it flat over the the part that needs the gasket and use the ball side of a ball peen hammer and tap around the edges lightly. It will form to the part and all you need to do when done is carefully rip off the excess material or use a razor blade.

A good replacement gasket material in a pinch are those manilla folders. Works with the above method as well.