You get a heating source (probably UTH) under only 1/3 of the tank. This way your snake can choose between the cool and warm side.

What are the temps in your house? This can really destroy your temperatures.

What I always recommend is this: You get a thermostat, and plug the hot side heat source into that and set it so that the hot side temp is ~90*. In the same power strip, you put a piggy back dimmer switch, and into that you stick the cool side heating source -- only necessary if your house temps are too cool -- which should cover the remaining 2/3 of the bottom of the tank. You dial the dimmer down till that side is ~80*.

Hope that helps!