How To Install A Masonite Prehung Exterior Door

The first thing you need to do is check the opening you'll be hanging the door in for the correct size. It should be 2" bigger than the door size. Even though it's a rough opening it should be reasonably plumb and square.
If the opening was framed by someone else, you may want to break out your level and framing square and check this also. Drywallers sometimes believe the rough opening was meant for them and will let the drywall run into the opening. If this is the case use a drywall saw or sawzall to cut it back.
Once all the vitals have been checked your ready to hang a door. Prehung doors come assembled a couple different ways. They can be bought with trim already mitered and nailed on to one side and without trim. If there is no trim installed, I like to put it on before I put the door in the opening. The trim is installed on the hinge side.
Most doors open into a room and against a wall. When putting the door into the opening, try to put the door in the center of the opening. The door jamb should be able to move to the left and right in the opening. The gap between the door and jamb on the hinge side is usually about an 1/8" of an inch. This dictates the gap or space you should have all around the door. Move the door jamb to the left or right until you have that same space at the top. You then nail the trim on the top hinge side and the bottom hinge side. Then nail the strike side on the top making sure you still have an equal space. Nail off the rest of the hinge side with 3 or 4 more nails. The 2 nails already in the top are all I usually put in. Now nail the rest of the strike side starting at the top and working your way down, maintaining the same space as the top and hinge side.
Once the door is nailed into the opening on the inside, it's time to shim the door jamb. First, pull the door closed to make sure it hits the door stop evenly on the strike side. If it is hitting only at the top pull the hinge side toward you till it hits even. If it hits only at the bottom, push the hinge side jamb away from you till it hits evenly.
Once you get the jamb aligned put shims between the jamb and stud opening, being careful not to bow the jamb into the opening. If need be use a straight edge to keep it straight. I put shims behind every hinge and the strike and also at the top and bottom of the strike side. I nail these shims in with two nails, one on each side of the stop.
The next step is to apply the door casing to the outside of the door. Once this is done, your ready for the door hardware. If everything went right, the bolt should engage the strike plate and the door should fit snuggly against the stops.