Measure the door opening. Transfer the measurement to the door so you can determine how much material you to remove from the door.
Use a square to draw a straight guideline for making the appropriate cut.
Place the bi-fold door onto a pair of sawhorses. Move the door so the cut line does not interfere with the sawhorses.
Ask a friend to hold the door. Turn on the circular saw and line up the cutting guide with your line. Move the saw slowly through the bi-fold door to cut the door to size.
Sand the cut edge with sandpaper to smooth the door and remove any burrs.
Replace the door into the opening.