You can wind up with a door with uneven gaps around it that does not latch correctly if you do not hang a door properly. Most installation problems happen because the floor isn't level beneath the doorway. If the floor slopes slightly and the jamb isn't trimmed to compensate, your latch won't line up.
Make sure to always check the width of the opening before you order your door. A good rule of thumb is to allow the width of your opening to be 2 to 2-1/2 inches wider than the door. You will want this extra space to fit the jambs plus the shims into the opening to install the door. You will need a 30-inch pre-hung door if your rough opening measures 32 inches.
The next step is to check the length of your pre-hung door-jambs. You might have to trim both sides so as to minimize the space beneath the door if they should turn out to be too long. You will have to minimize space and both sides of the jamb may have to be trimmed should they turn out to be too long.
The vertical sides of your opening must line up exactly to an even horizontal line either vertically or as perpendicular to it. Although some variations from plumb are acceptable, if the openings have a trimmer stud more than 3/8 inch out of plumb from top to bottom you will not be able to install the door correctly.
Before installation, measuring and cutting the door must be done carefully and accurately. You will need some simple carpentry tools and some basic home improvement abilities to easily master the procedures. The first door will probably require you about one and one half hour to install, but after that you can plan on a second door being done in half the time.
If a door closes tightly and quietly and doesn't swing open you know that it has been correctly installed. A well-installed door has been trimmed in such a way that there's a very small gap of just below one-half inch next to the jamb. Where the hinges sit or the lockset slides great care should be taken when cutting the various holes or slots in the wood. If the cut is too deep, the door won't shut properly and if it is too shallow, whenever the door moves it will creak and scrape. To create the perfect mortise, a sharp chisel and a steady but light hand is necessary.