How do you seal a fireplace insert to the brick?

Well if your talking a gas fireplace it is done the same as a hot water tank.

You hold down the gas button and light the pilot and hold button down for one minute

and then release the button ,if the pilot stays lite your set if not you need too do it again. If you hold the button down for 3 minutes and the pilot does not stay lite then the thermocoupler is bad.