Step 1
Measure the rough opening of the door in the brick, and measure the pre-hung door casing. Select boards of treated wood that will be used to create a wood structure, fit to fill the size difference between the rough opening and the pre-hung door. These boards will flesh out the rough opening and create a frame the door can fasten to.
Step 2
Cut the treated wood planks you selected in Step 1 to the size necessary to create a frame in the rough opening. Use a circular saw to cut the planks lengthwise, and a miter saw to cut the ends so they fit together at 45-degree angles in the corners of the opening.
Step 3
Screw the treated wood into place in the rough opening, using cement screws, if the interior of the walls are cement blocks. If the interior of the rough opening is wood, use wood screws. Level the sub-sill with a level before screwing the sill into place. Insert shims under the sill if necessary, to create a level surface for the door to sit on.
Step 4
Run a bead of caulk along the sill and the frame. Tip the pre-hung door case into the rough opening. Screw the door case into the wood frame according to the manufacturer's instructions.
Step 5
Seal any spaces between the wooden frame and the exterior brick with caulking, then seal the door from the inside. There will be a gap between the door casing and the wall. Insert expanding foam insulation between the door frame and the wall, then fill any remaining spaces with fiberglass insulation. Cover the exposed insulation by nailing trim into place.