Measure the opening of the fireplace. Take a measurement at the front of the fireplace, from the left side of the opening to the right side of the opening. Take another measurement at the back of the fireplace from left to right. Take a measurement from the front of the fireplace to the back wall. Use the measurements to order an insert.
Run a gas and electrical line to the fireplace area. It is required by law that this be done by a certified professional.
Remove the screws holding the chimney cap on the chimney. Climb to the roof and unscrew the cap. Slide a knife under the metal to break the seal between the metal and the brick and remove the cap.
Measure the size of the exhaust stovepipe thimble and cut a square hole in the wall inside the fireplace. Attach the metal covers together, one inside and one outside, to make a hole for the stovepipe thimble.
Slide the stovepipe to the back of the fireplace insert and rotate the pipe until it locks.
Slide the fireplace insert into the fireplace hole so that the stovepipe is against the stovepipe thimble.
Go outside the home and slide the termination cap into the thimble. This seals the hole and the stovepipe on the inside.
Call a professional to have the gas attached to the fireplace insert and to have the installation inspected. This is often a requirement.