While replacing a door into an existing opening requires few tools, it does require patience and care. Most interior doors have hollow cores and can be unforgiving for certain types of mistakes.
Remove the old door and cut the new door down to size. If you have more than 3/16” of an inch to remove, you can use a circular saw to do so. If you’re not steady with your saw, you can use a straight edge as a guide. Go slowly and make a long, clean cut. If you have more than an inch to remove, take half off the top and half off the bottom.
With your chisel’s beveled edge flat against the door’s edge, gently tap to remove material within your score marks. You want to make a nice flat surface for your hinge to lay into. Go slowly and test-fit your hinge a few times until you get it mounted flush. If you accidentally go too far, you can place a thin wooden shim under your hinge to bring it back flush with the edge of the door. Repeat the mortising process for the other hinge along with the lock set.
Next you’ll need to drill the hole for the door knob and the lock set. Using a hole saw that is the correct diameter for your knob, gently drill out the hole for the knob. Then, using a spade bit, you’ll drill a hole through the lock set mortise into the knob’s hole. Make sure not to penetrate the back edge of the knob hole. At this point, you can apply paint or finish to your door if desired.
Hang your new door and test-fit everything. If your door sticks, make a mark on the edges where it’s not closing flush. You’ll need to take the door back down and shave some material off using your handplane until the door opens and closes freely. If your knob and lock set are loose, you can bend the striker plate outward slightly, or unscrew the knob and make any necessary adjustments until your door is opening and closing freely.