To make perfectly square crosscuts with your circular saw, build a simple T-shaped crosscut guide. Start by cutting a 1×4 fence 18 to 20 inches long; that’ll allow you to crosscut boards up to 12 inches wide. Next, cut a ½-inch plywood cleat 3 inches wide x 16 inches long. Glue and screw the plywood cleat to the 1×4 fence, using a framing square to ensure the two pieces are at exactly 90 degrees. Now, before using the guide you need to trim the cleat to size: Simply run the saw along the fence to trim off one end of the cleat. Now, to use the guide, mark the cut line on the board, align the cut-end of the cleat with the mark, then simply guide the base plate of the circular saw along the fence to make a perfectly square cut.