To install an exterior door, remove the old door, and apply a bead of caulk on the outer edge of the sill, 1/2 inch behind the sill and between the sheathing and door casing. Set the new door into the opening, use a level to ensure the door is plumb, and hammer nails into the pre-drilled holes in the casing to secure the door. Place shims behind the hinges to fill any gaps, and use screws to secure the door.
When securing the door with screws, replace two of the short screws at the top and bottom of the door with screws that extend into the door jamb by 1 inch, and drill the screws through the shim placed behind the hinge. After the door is secured, apply a bead of caulk around the exterior door casing, and fill nail holes with wood putty.
To install a prehung door, remove the existing door, door trim and door frame, and apply a bead of caulk around the subsill. Secure the top and side of the door with shims, and use a level to ensure the door is plumb.
Insert shims behind the hinges, and drill screws through the hinge and shim to secure the door. Fill gaps between the door frame and subsill with spray insulation, and reattach the trim.