Hi, Robert. The way I do it is to take down the door and lay it on 2 saw horses. Put masking tape across the bottom of the door on both sides to minimize splintering. Measure down from the top to calculate how much you want to remove and make a mark on one edge. Now do the same on the other edge. This will keep your cut parallel. Measure from the side of the blade on your circular saw to the edge of the base on the wide side. Make marks up this distance from the first set of marks. Clamp a straight 1x board across the new marks. If you did it right, your saw blade should now line up with the first set of marks. Make the cut with the saw base sliding along the edge of the 1x to get a nice, straight cut. Thge wide side of the saw base should be on the door. Remove the masking tape and rehang the door.