Hanging doors can be a great do-it-yourself project. Most pre-hung doors will come nailed closed with a header nailed through the jambs.
Remove these nails (usually two) and kick off the temporary shipping brace across the bottom of the doorjamb legs. Check to make sure that the staples on this brace also come out or it will be hard to level the door. For beginners, it will be easiest for you to now case one side of the doorjamb (case the side with the hinges showing).
Run the casing across the top first, then down both sides to within 1/8 inch of the bottom of the jamb. Running it a little short will keep it from interfering with you while you are trying to level the door.
At the rough door opening, snap a line on the floor across the opening of the door, even with the outside face of the drywall. This creates a baseline that ensures a plumb and perpendicular installation of the doorjamb. Now come to the stud, which will be on the hinge side of the door.
Out from this stud, draw a two-inch perpendicular line to the line you snapped. This will be a visual aid in shimming your door to ensure you do not rack the jamb or hang the door in a hinge-bound condition. With the door closed and cased on the hinge side, set it in the center of your rough opening. Place a two- or three-inch level on top of the casing and shim the jamb legs off the floor as necessary for a perfectly level installation of the head of the door jamb.
Place two finish nails in the head casing; do not completely set the nails at this point. Now proceed down the hinge leg of the door, checking for a level and a consistent reveal between door and jamb. Place one finish nail in the casing somewhere near the lower hinge, driving to grab the rough stud, but do not completely set the finish nail.
Repeat the procedure on the strike leg of the door. Your door is now temporarily set and can be opened and closed to verify that it is not dragging on any of the jamb and that the reveal is consistent when closed.
Now go to the hinge leg of the door; check your lines on the floor to ensure the jamb leg is parallel to the second line you drew on the floor. We will now shim the jamb and secure to the trimmer stud with six, 8d-finish nails. Be careful in using the shim shingles that you do not skew the jamb out of plumb and square. Repeat the process for the strike leg of the door.
Now remove the top screw in the top hinge of the door on the jamb side. In this screw hole, pre-drill and insert a two-inch wood screw through the jamb, through the shim shingles and securely into the trimmer stud.This will hold the door plumb and secure for years to come and will keep it from ever sagging.
To complete the installation, now install the casing on the other side of the jamb.
