The first thing that you need to do is actually get the door that you want to install. If you've already done this, you can skip ahead to the next step. If not, you should start by using the tape measure to get measurements for the width of the opening, the height of the opening and the thickness of the wall that you will be installing the door in. Make sure that you take the measurements at least twice to eliminate errors, since a prehung door can not really be changed once you've made the purchase. When you have completed the measurements, all that is left to do is take them to a store that sells prehung doors and then get a door with those measurements and a style that you like.

Once you have the door, it is time to get it ready and in a position where you can begin to attach it to the wall. In order to do this, you need to start by placing the door in the opening and then using shims to help you hold the door in place. At this point, you should try to make sure the door is centered in the opening as much as possible.

Once that is complete, the next step is for you to take some more shims and use them to fill in any spaces that exist between the frame on the prehung door and the framing on the jamb where you will be positioning the hinges as well as the lock. Make sure that the space is filled in completely, otherwise the door will become misshapen when you attempt to start screwing it in.

With that done, your next task is to make the position of the door completely plumb. You can use a carpenter's square, a level and any other tools you can think of to help you in your task and then you can use whatever shims you have left to hold the door in the plumb position you find for it.