Lay the door out on the sawhorses with the bottom edge toward your power source.
Measure and mark the height you need to remove; check your measurements again before proceeding.
Score the length of your cutting line with a utility knife, using a straightedge to ensure a straight, clean score.
Turn the door over and repeat the measuring and scoring process on the opposite side of the door; this will prevent splintering when you cut the door.
Cut down the door on the marked and scored line, using a circular saw and taking care not to move above the line.
Sand the cut edge slightly to remove any burrs and roughness. Your door is now ready to rehang.