Lay the stretcher bars on a flat surface and join them at the ends to create your frame shape. There are grooves on the ends of the stretchers that fit together.
Stand your frame upright and and carefully tap on each corner with your hammer to better secure the stretchers.
Check the angle of your one of your corners by holding the right angle over the outside of one of the corners. Bend and twist the stretchers until the corner measures at a perfect right angle. Once you've got the first corner right, turn the canvas and measure the next corner. Measure all four corners and make necessary adjustments.
Put one staple in the middle of each of the corners so they don't lose shape when you stretch the canvas. This should be done on the back of the frame.