Your opening should be square and level, but it never hurts to check it with a level. It may not be perfect; that's why it's called a rough opening. If it’s out of square, you’ll need shims to install the prehung door straight and plumb. There should be a variance of less than 3/8” from top to bottom and side to side. Fit your door into the opening and see if you need to trim the jambs. You want the door to clear your floor by about a half inch. If the floor is not level, don't try to square the door to the flooring. It will never hang or close correctly. Cut the jambs to fit the floor. You can trim the bottom of the door to conform to the floor later. Measure very carefully, because you don't want to trim the jambs too short. Half the trick with interior door installation is measuring and cutting carefully.

Set Your Prehung Door
Push the door into the opening, it should stay put. One problem you will encounter is the door swinging freely while you install it. You cannot take the door off the hinges, because you will need to keep checking that it is opening and closing correctly throughout the installation. You can nail some scrap lumber diagonally across the top corners of the opening to prevent the door from swinging wide open as you work.

Shim Your Prehung Door
Starting at the top, tap your tapered shims between the framing and the jambs. Generally you want shims at the top, bottom and about halfway up. This will take some adjusting. Close the door and check the gap all the way around. Since the shims are tapered, they can be pulled out or tapped further in until your door is hanging straight. Open and close it, checking for a smooth swing. Once it is even all the way around, double-check everything with your level. Use wood shingles for shims rather than the molded plastic shims. These are much easier to nail.

Nail in Your Prehung Door
Starting on the hinge side, nail the jamb to the frame. Start by nailing through each shim. You need to make sure that the door jamb doesn't twist out of square as you nail it to the frame. After each nail is set, close the door and double-check the gap and how it meets up to the door stop. Once everything is set solid, use a utility knife to score the shims flush and break them off. Now you are ready to install your trim and lockset.