(Reprinted from www.hometips.com)

With a factory-manufactured pre-hung door, hinges attach the door to the preassembled jambs, and the sill and jambs are braced to keep the whole assembly square until you install it. Pre-hung interior doors normally have a piece of scrap trim stapled to the bottom of the door and jambs. After carrying the door to where you intend to install it, remove this temporary brace. (You must be able to open and close the door as you place shims between the studs and jambs, drive in nails to hold the jambs, and check the alignment of jambs.)

When you are ordering a pre-hung door, make sure you specify the wall's overall thickness. The jambs ordered for plaster-covered walls are different than those for walls finished with drywall. When you fit the jamb into the rough opening, you will see that the jamb is wider than the rough framing and that it sticks out on either side. Be sure the distances are proper to allow for drywall, plaster, or--in the case of an exterior door--sheathing and siding.

Position the Door Properly

Place the door in the rough opening, centering it in the frame and fastening it as discussed in the steps shown here. Using a piece of drywall (or other finish material) as a spacer against the trimmer studs, slightly adjust the unit until it is flush with the spacer. (If the finish floor has not yet been installed, raise the side jambs to the correct level with blocks; you want to avoid cutting off the bottom of a new door if possible.)

Before nailing the jambs, it's very important to make sure the door fits squarely in the opening and that the jambs are plumb, both from side to side and from front to back. Drive pairs of tapered wooden shims between the jambs and the trimmer studs (if the casing is attached to the jambs, insert shims from the open side) to adjust the unit and hold it in place until you nail it.

Secure the Door Hinge-Side First

Start by shimming the lower hinge side of the door. Nail through the jamb and shims partway (1 inch) into the stud with a 10d finishing nail; position the nail where the stop moulding will cover it. Insert shims next to the upper hinge location, check the jamb for plumb, and nail partway. Again, shim, plumb, and nail halfway between the top and middle hinge positions. Repeat this process between the middle and bottom hinges. Check to make sure the jamb above the doorway is level. Now shim the opposite jamb at similar locations, but do not nail where you will need to cut for the latch.

Complete the Installation

Remove any bracing or blocking tacked to the unit. Close the door, and check that there is the same amount of space (1/16 to 1/8 inch) between the edges of the door and the jambs. If the door sticks or is out of alignment, pull out nails in the area that seems to be the problem, using a block to protect the jambs from your hammer. Adjust the shims and re-nail. Install the lock set. Drive the nails almost flush, and then set the heads with a nailset. Using a handsaw, cut off the shims flush with the jambs (you can just break off short, thin pieces). Finish with door casing or other trim.

For exterior doors, nail the threshold between the jambs, shimming below if the threshold does not rest securely on the subfloor. Finally, install the stop moulding with 4d finishing nails.

1. If you're replacing an existing door, pry off the old trim, and pry out the side and head jambs. If you're installing a door in a newly framed opening (click here to learn about cutting in a doorway), be sure the framing is sized properly.

2. Set the pre-hung door into place with the hinges positioned on the proper side, and tilt it up into the opening. It's a good idea to have a helper on the other side of the door to receive it and help shift it so that the jambs are flush with the walls.

3. Push wooden shims between the jambs and trimmer studs on both sides while checking the jambs for plumb with a level. Tap the shims with a hammer until they're snug.

4. Nail through the jambs and sets of shims with 10d finishing nails. To avoid denting the jambs with the hammer head, stop nailing before the heads reach the surface and finish with a nailset.

5. Install new trim around the door, using 6d (2-inch) finishing nails.