Choose your door. The easiest way to install a new door is to use a pre-hung unit where the door is already attached to the frame.
Check the instructions on your door to determine how large the rough opening size should be. If no dimensions are given, assume that the rough opening should be one inch taller and one inch wider than the door frame.
Mark the locations of the new door on your wall using a pencil and a straight edge. Think carefully about the location before you choose it. You'll want the door to swing freely without hitting anything or impeding traffic through the home. You'll also want to avoid plumbing so add the door in a spot that is not directly above or below a bathtub, sink or shower.
Use a sawzall or utility knife to cut away the drywall on one side of the new opening. Cut only the drywall and avoid cutting too deeply into the wall cavity. Examine the inside of the wall to see if there are plumbing or electrical components in the way.
Cut the drywall on the other side of the wall away along with any studs that are located within the opening using a reciprocating saw.
Add studs on either side of the opening to frame out the door. Use 2x4 or 2x6 lumber to create these studs, and nails them in place alongside existing framing members. Make sure to maintain the required rough opening size plus a one inch clearance all around.
Slide your pre-hung unit into the opening and add a couple of nails to each jamb to hold it in place. Use wooden shims under the legs of the frame or between the frame and the wall studs to level the unit. Confirm that the unit it level before proceeding.
Add screws or nails along each jamb to secure the unit in place. Make sure to add fasteners to any shims that were used. Add trim or casing to cover the joints between the frame and wall. Repair the floor under the opening or add a threshold to cover any gaps.