Installing a prehung exterior door involves preparing the door opening for the door installation, installing the door frame in the opening and attaching the door onto the frame with hinges. Sealing and insulating the door completes the process.
To prepare the door opening for installation, remove any old door from the opening by releasing the hinge pins to detach the door from the frame, and set the door aside. Follow up by prying the old frame from the door opening and scraping old caulking off the opening. Adjust the dimensions of the opening as needed to suit the size of the door you intend to install by removing part of the wall, taking care to ensure that the opening allows the door to fit in the opening plumb and level.
To install the frame, measure and cut lumber according to the dimensions of the opening, assemble the cut pieces into a frame, and secure it onto the door opening with cut nails, taking care to adjust the frame to make it plumb and level. Once the frame is in place, caulk the opening's bottom subseal, set the new door into the frame, and adjust door's position using a plumb line and a level. Attach shims, and nail the door in place. Cut away extra shim parts, test the door, and secure it on the frame with hinges. Finally, apply caulk to seal the door, and attach fiberglass insulation.