Select four stretcher bars, two of each desired length. For example, if you want to build a 16-by-20 frame, you will need two bars that are 16 inches in length, and two that are 20 inches in length. (If you are using a bar greater than 40 inches, you will require a cross brace to prevent warping.)
Lay the stretcher bars on a level surface, making sure the beveled edge, or ridge, is facing up. Align each matching length parallel to each other to create a square or rectangle, according to the chosen dimensions. If a cross brace is necessary, place in the middle, perpendicular to, and bracing the two longest sides.
Join the corners of each stretcher bar by sliding the overlapping tabs into their corresponding slots. If using cross braces, insert these into their appropriate slots. Repeat for each corner. Tap corners into place with your mallet.
Use the t-square or right-angle tool to check that each corner is set at a 90-degree angle. If not, use the rubber mallet to gently tap into place. It is important to verify at least three corners are set at a right angle. The fourth corner will naturally fall into place.
Carefully turn stretcher bars over so that the beveled or ridged edges face down. Double check right angles with the t-square or right angle tool. If correct, staple each joint two to three times, which will hold the bars in place.