Cutting interior doors is not the same as cutting exterior doors. Exterior doors are usually solid core and are much heavier than the interior hollow core doors. There is only about 1 inch to play with, so you must be careful when cutting the length of the door. The rails at the top and the bottom of the door are only 1½-inch thick. Proceed with caution.
Measure from the top edge of the door and make several marks across the bottom of the door representing where the door will be cut. Place the straight edge on the marks and score a line with the utility knife.
Measure the distance between the blade and the right-hand edge of the circular saw's plate. Measure from your scored line and make several marks across the width of the door. Keep in mind that these marks will have to be removed after the door is cut.
Place the 36-inch straight edge on the line and clamp it to the door with the C-clamps. Next, cut the door with the circular saw keeping it firmly against the straight edge.
Sand the cut edges on both sides of the door. Be careful not to sand the door beyond the cut edge to prevent sanding marks across the grain of the wood. Go to Step 5 and 6 if your cut removed the bottom rail of the door.
Remove the honeycomb cardboard with the utility knife. Be certain to remove enough to allow the insertion of the 1¼x1¼ piece of lumber. In some cases, the strip of wood may need to be sanded because it needs to be a tight fit.
Once it fits, apply glue to the sides that will come into contact with the front and the back of the door and slip it into place. Then, clamp the two 36-inch 2x4s over the new strip with the C-clamps. Remove the clamps when the glue is dried and touch up the edge with sandpaper.