Install Prehung Door with Flat Jamb
Remove braces and nails using a claw hammer.
Place the closed door in the rough opening and center it. Use a level on the hinge side of the casing to align it vertically. Tap shims into place between the door jamb and the rough framing to make the jamb plumb.
To hold the door in place, nail through both the jamb and the shims into the wall stud near the top hinge using an 8d finish nail. Any nails that you drive through the jambs should also go through shims behind the jamb. This is necessary to prevent distortion of the jamb in the nailed area, which could cause the jamb to bow outward at the nails.
Do the same for the other hinges.
Check the door's alignment with a level again before continuing around the door jamb, installing shims and nails in the same manner. The space between the jamb and door should be the same for the entire length of the unit.
Trim any shims that protrude from between the jambs and framing to allow the door trim to be installed. Be careful to avoid damaging the jamb or wall.
If necessary, miter the door casings and nail them in place. Nail every 16 inches using 4d finish nails. Set the nail heads.
Install the doorknob. The holes for the knob and latch assemblies should already be cut for you.