- Cut out four 6-in. squares from 3/4-in. plywood.
- Use a jigsaw to cut 1-in. wide notches at a 45-degree angle into one corner of each of the four squares. You’ll use the notch to check the underside of the joint so you can wipe off squeezed-out glue before it hardens.
To use, clamp one mitered frame piece down tightly to the ﬁrst square, apply glue and press the second piece into place, snugging it up tightly by hand so the glue oozes out along the joint line. Adjust until the joint aligns perfectly; then tighten the second clamp. Adjust, glue and clamp the second miter using another plywood square.
After gluing two joints, you’ll have a U-shape formed by three frame members. Cut the fourth frame member a little oversized, test-ﬁt it and trim it for a perfect ﬁt with a miter saw or disc sander. Glue it into the frame using the last two plywood squares. The out-of-the-way clamp positions allow you to drill pilot holes and drive brads into each corner to reinforce the joint if you wish.