Okay... before we knew about using UTHs with a thermostat we used just bulbs because we did not trust them. We use to do day/night switch and don't anymore.
You can use the day bulb during the day and the night bulb at night. In a 15 gallon tank both lights would most likely be waaaaay to much heat and you would not have a thermogradient in the tank allowing him to go where he needs to be for the correct temps.
Without the thermostat I would NOT use the UTH
Increase the temps in your home to 72-75
Place the red bulb in the light hood and place on one side of the top.
Place the hide under the direct center of the bulb. This will make it so that he can climb on top for even warmer temps if he needs to.
Place the water dish on the cool side close the the center of the tank.
Place a hot, damp towel over the top 1/2-2/3 of the tank on the cool side. This will help keep the heat inside of the tank.
With me so far?