Close the garage door. Place the pointed end of the compass on the ground at the widest point of the gap. Expand the compass until the scribe side touches the bottom of the door and tighten the compass screw.
Move the compass to the side, keeping the scribe parallel with the compass's pointed end. Allow the scribe to mark the garage door. Continue until you have covered both sides of the garage door.
Open the garage door halfway. Turn on the jigsaw and cut the garage door along the scribed line. Sand the cut edges down until smooth.