Position the rocks at the basking end of the cage, the hiding place at the cooler end. An under-tank heat tape or hot rock in the hide place may be useful to keep the night time temperature warmer. Fine substrate like beach or play sand is probably the best. DO NOT position heavy rocks on loose substrate. The lizard will dig under the rock and may become pinned and crushed.
Collared lizards dig and controlling the location of their digging is helpful.
Sloping cork bark or rocks piled high under the light bulb will allow for easier temperature regulation by allowing the animal to move closer or farther from the light source. This may also allow for a smaller wattage bulb by allowing the animal to move closer to the light source. Collared lizards are very active. It's normal to see them climb the walls of the enclosure or jump up onto the screen and hang upside down. They should be allowed to perch on the hand or fist of an outstretched and raised arm.