Run a tape measure along the width and height of the rough opening and record the measurements so the exact size door can be purchased.
Clean around the opening and cut back any excess shims and drywall with a hacksaw. Run a level vertically along the hinge side where the door will be installed to verify that it is square and plumb.
Hold the door by the brick molding and tip it into the opening from the outside of the house. Open the door and step inside. Shut the door and verify that the gap between the header along the top of the door and the jamb is the same along the entire length.
Add shims just above the hinges between the jamb and the frame. Insert a nail with a nail gun at each shim location into the jamb to secure the door in place. Repeat the process for the other side. Screw the hinges into place with 2-inch wood screws.
Close the door and check for any gaps. Cut off the shims with a utility knife to remove the excess. Nail the brick molding to the house along the outside by inserting a nail every six inches across the top and along the sides. Open and close the door to make sure it operates correctly.