Like Dan said fill the bottom of the vivarium 1/3 with the heat mat.
Hook it up to a stat too, very important, controls the heat.
You also need a thermometer, get a digital one. Infact get two, one for the hot side one for the cool side, so you can keep an eye on the temps.
1.0.0 corn snake - Hiss (ghost) 0.1.0 Corn Snake - Mars (Amel) 0.1.0 Leopard Gecko - Mouth ( super Hypo carrot tail) 1.0.0 Chinese Hamster -Snowball 2.0.0 Gerbil - Ebony + Caramel 0.2.0 Fancy rat - FiiFii + Ruby (adoptions)
RIP Milksnake - Kaa, Russian Hamster - Star, Chinese Hamster - Gok