Using a hammer and a flat screwdriver, remove the hinge pins and lift out the old door. Unscrew the hinges from the door.
Next, unscrew the hinge leaves and remove them from the jamb; keep the old hinge sets together, including all screws, hinge leaves and hinge pins.
Using a utility knife, score the caulking between the molding and the brick to break the seal. Use a pry bar and hammer to carefully remove the molding. Pry away the door jamb, framing and threshold.
Finally, scrape away old caulking.
To calculate the exact dimensions of the door opening, measure the width between the side jambs, the head jamb to the seal under the old threshold, and the thickness of the wall. The rough opening should be at least one inch wider and taller than the outside frame dimensions of the pre-hung door to be installed. Be sure to measure carefully.
With the shipping brackets still attached, carefully set the door in place, centering it in the opening. If necessary, add shims under the lower side jamb until the entire unit is plumb and level. With a carpenter's pencil, mark a line at the top of the door on the brick lintel, then carefully remove the door.
Use the level to mark plumb lines on both sides of the opening. These lines will serve as guides for the molding of the new door. Remove any remaining pieces of paneling around the door to ensure a clean opening.
For a very rough opening -- one that has concrete or cinderblock as the jamb -- pressure-treated lumber may be necessary to frame out the door. For this, take the measurements for the side jamb and cut the board to length. After checking for plumb, install the board using cut nails designed for fastening to masonry. This board will act as additional framing for the new door.
Begin installation by first running a thick bead of latex caulk along the subseal at the bottom of the old door opening. Then lift the new pre-hung door into the center of the rough opening, positioning the bottom seal first, then tilting the door into place. Check for plumb and shim the frame as necessary.
Secure the door with cement screws at each shim location. For wood frames, use casing nails. Trim off excess shims using a utility knife and pry bar.
Remove the shipping brace and test the operation of the door. Finally, attach the long-anchor screws (provided with the door) through the hinges and into the door framing.
To seal the exterior of the door, run a bead of paintable latex caulk around the entire door. Fill holes left by the screws with wood filler.
Inside, insert fiberglass insulation into the voids between the door frame and opening. Attach trim to the door with fourpenny finishing nails.
Once the caulk and wood filler have dried, you can apply paint to the door and wood trim to match the color of your home. Finally, attach the door handle and deadbolt to the new door.