If it is a glass tank you place the UTH outside attached to the bottom of the tank if it has that plastic base that keeps it off the ground, the UTH plugs into the thermostat with a probe and you then place the probe on the inside the tank on the glass above the UTH(foil tape can be used to hold in place) and then set the temp on the thermostat. Your BP will find the heat that he likes and will get comfortable the bottom, ideal temps would be 88-90 on the glass. The UTH will do little to keep the ambient temp in the cage up so don't freak out if your temp gage reads low on the "hot side" , as long as it stays in the high 70's to 80's for the ambient temp in the cage he will be fine. A temp gun can be a useful tool as a quick way to ensure your hot spot is on point.

If it is a plastic tub you can keep the UTH of flexwatt on the "floor" and not attached to the bottom of the tub. Remember that the flexwatt is heavily influenced by the air temp surrounding the cage, for best results you want to keep the room temp closer to the ambient temp of the tank. If you have the tank in a room that is in the 60's with the AC on all the time, it will be constantly running trying to keep the temps up and you will likely need another heat source to keep the ambient temps in the tank up.