Hold the door into place. Look around the edges to determine if you need to shorten the length or width.
Measure the closet door and the closet door opening. Mark the door where you need to cut it so it fits into the opening.
Place the door onto two sawhorses. Place towels beneath the door to prevent damaging the finish.
Mark a guide line on the door where you want to cut it using a straightedge.
Cut along your guideline using a circular saw. Move the saw slowly to prevent jumping the line and creating an uneven cut.
Smooth the cut edges using sandpaper. Hold the door in place to ensure that the cut allows the door to fit properly.