This story shows how to install a prehung door.
A prehung unit takes a lot of the demanding precision work out of installing a door. Hinge mortises and holes for the lockset and strike are already cut. But you still need to work carefully for best results. Check that the rough opening is fairly square and plumb. As you work continually double-check the unit for square and plumb. Take the time to fix mistakes as they occur rather than hoping they will be hidden by the next step.
If you are hanging a door in an older house, keep an eye on the big picture. Before setting the nails step back and look at your work. If the wall or floor is off level or plumb, you may want to align the door frame at least a little bit out of plumb in the same direction so it blends in. For example, if the wall leans slightly, match the jamb to the wall. It won't be noticeable as long as the door doesn't lean enough to open or close by itself.
About 2 hours per door
Level, circular saw, layout square, hammer, nail set, utility knife
Cutting, driving nails, checking for plumb
Frame the doorway and apply drywall to both sides.
Prehung door to fit the opening, shims, 16d finishing nails, 8d finishing nails
If the hinge-side stud is not plumb, nail either the top or bottom, whichever is closer to the center of the opening. Insert shims at the opposite end to plumb the jamb. Below the shims, drive two 16d finishing nails just far enough to hold the shims and jambs in place. Adjust shims if necessary.
Before setting nails, check that the hinge jamb is centered across the wall thickness. A typical jamb is slightly wider than the wall thickness to allow for irregularities in the drywall. If adjustment is necessary, pull the nails, protecting the jamb with a scrap under the hammer.
Put the door back on its hinges and swing it closed. Insert shims between the jamb and the stud about halfway between the hinges and adjust them until the gap between the door and jamb is equal from top to bottom. Open the door and drive two 16d finishing nails below the shims.