Measure the width of the door opening and cut the slab to that size. If you have a casing around the door opening, hold the slab in place and trace around it. Take the piece that was cut off and use it as a template for your cuts on the side jambs.If there is no casing, measure from one edge of the door opening to another, subtracting 1/8" for clearance, and mark that measurement on each jamb. Cut along those marks using a circular saw or handsaw. Assemble the door frame using construction adhesive and 2-1/2" finish nails into the framing studs.Attach hinges to the door and screw them into place on the jamb.

Installing an interior door is a fairly straightforward task that can be completed in a few hours. Here are the basic steps:1. Remove the old door from its hinges and remove any hardware from the door, such as the knob and screws.2. Measure the width of the opening and cut a piece of wood to that size using a circular saw or table saw.3. If you're using a pre-hung door, attach the hinges to the door using screws. If you're using a slab of wood, attach hinges to both sides of the wood at the top and bottom. 4. Hang the door in the opening and make any necessary adjustments to ensure it's level.