To replace an interior door, measure the height and width of the old door and use these measurements to purchase a door blank. If necessary, choose a door blank that is larger than the old door and trim it to size. Door manufacturers don't specify door widths by inches, instead they use a notation that specifies the width of the door in feet and inches. For example a 3-0 door is 3 feet 0 inches wide, not 30 inches wide.
Remove the old door by removing the pins from the door hinges. Always remove the pin from the bottom hinge first, and ask someone to hold the door as a safety precaution. Remove the hardware from the old door.
Place the old door on top of the new door, and use the old door as a template to mark where to install the hinges and door handle. Mark the location of the hinges using a utility knife to make a score at the top and bottom of each hinge along the door edge. Also, mark any areas where you need to trim the new door blank to fit the existing door frame.
Use a saw to trim the door down to size or use a hand plane to make small alterations. Chisel out the area between each pair of hinge score marks to accommodate the hinge. Install the hinges on the door. Hang the door by placing the new door into the old door frame and replacing the hinge pins. Check for any fit issues, and make any necessary alterations with a hand plane or chisel.
Remove the door from the frame. Drill out the section of the door to accommodate the door knob. Install the door knob, and rehang the door.