1. Buy your door.
Purchase a door to fit the rough opening in the wall. Doors and door openings are generally standard sizes, generally 24-36". The framed-in rough opening for the door is always around 2" wider than the purchased door (not including the jamb around the door). This allows for adjustment when installing the door to achieve plumb "level".
2. Start getting things straightened out.
Start by drawing a plumb line on the wall.Measure 1/2" in from the rough opening on the hinge side of the door. Using a 6' or 4' level draw a level line down the drywall. You can also use a laser level, which may be easier and more accurate (there are many models that can attach to a wall).
3. Attach the door installation brackets.
Attach 6 door installation brackets to the outside of the door jamb, the wood frame to which the door comes pre-attached. Place a bracket behind each of the three hinges. Attach the remaining three brackets on the the other side of the jamb.The first bracket should be 8" from the top, the next bracket just above the latch stop, and the final bracket 8" from the bottom of the door.
4. Place the door into the opening on top of blocks or shims.
Place 1/2" blocks under the door if a carpet or hardwood is going to be installed or 1/4" blocks if installing laminate. Never place the door directly on an unfinished floor.
5. Attach the brackets.
Using the plumb line on the wall, screw in the top bracket on the hinge side of the door. Then screw in the next 2 brackets using the same reference notch as the first bracket. After the 3 brackets are screwed into the wall using the same reference notch for each, the door is now level. Now check the reveal (gap between door and door jamb) when screwing in each of the final 3 brackets. Start at the top of the door and check the reveal when screwing in the top bracket. Check the reveal when screwing in the final 2 brackets. The door will now be perfectly hung and the blocks under the door can now be removed.
6. Install the casing around the installed door.
The casing, also called the trim, is the strips of wood that hide the joints and some parts of the hinge. Done correctly, the trim will perfectly conceal the door installation brackets.Choose casing that compliments your style and cut it using mitered corners or other styles, depending on your preference.