Use a square to determine if the frame is at right angles. Also measure each side to be sure they are the same length. If they are not the same length, hold the support on the floor and with a hammer and a block of wood tap the longer sides until all four are equal. It is very important to spend time making the supports square now, not later when you begin painting or trying to frame the work.
On the back, put three staples at each corner, which will help keep the supports from becoming unsquare while stretching. However, if you later have to use wooden keys in the corners to tighten the canvas, these staples will make it more difficult.
Spread the canvas or linen on the floor and lay the supports with bevel in, or facing down, on the canvas, leaving at least 2 inches on each side. Staple or tack the canvas at the center on each side, trying to keep the grain of the canvas running at right angles to the supports.
Remove a staple on one side, start on one of the longer sides if the canvas you are stretching is not square, then using the stretcher pliers or your hand pull the canvas tightly over then staple. Rotate and do the next side following the same procedure. Now begin stretching the rest of the canvas, only add a few staples to a side then add the same to the opposite side. This will help prevent warping of the stretcher bars.<
When finishing the corners, the extra material should be folded neatly to ensure a good clean finish. The corners are the most difficult part about stretching and it may take some time to achieve neat looking corners, but with a little practice it can be done.
