Measure the length and width dimensions of the painting. Measure the dimensions of the painted portion of the canvas, not the perimeter. Cut two pieces of wood from the 2-by-1 1/2-inch board equal to the length of the painting -- minus 3 inches -- with a miter saw. Cut two more pieces equal to the length of the painted portion of the canvas, minus 3 inches. Adjust the blade of miter saw in relation to the saw's guide, angled to make 45-degree cuts. Lay a length of wood on the table of the saw against the guide, the 2-inch sides facing up and down.
Cut a 45-degree angle with the miter saw diagonal across the face of the wood from a corner at one end. On the other end of the wood, from the corner corresponding to the first -- as opposed to the diagonal corner -- cut another 45-degree angle. The cuts remove two small 90-degree triangles from each end. Cut the remaining three lengths of wood in the same fashion.
Wipe wood glue on the face of each 45-degree cut at the end of each piece of wood. The pieces of wood are glued together at the 45-degree cuts. Make a rectangle with the lengths of wood -- a square if the lengths and widths are equal -- by butting the 45-degree corner-cuts together. The face of each 45-degree cut on the end of each piece of wood butts up against the 45-degree cut of another piece.
Butt the inside corner of the carpenter's square up against each of the four corners of the rectangle frame. Adjust the legs of the frame until all four corners are square. Allow the glue to dry for 24 hours.
Draw a rectangle on the canvas around the painting -- 3 inches from the perimeter of the painting -- using the carpenter's square as a straight edge. Cut along the lines with the carpenter's knife to remove the excess canvas. Again, use the square as a straight edge. Place the painting face down on a smooth surface, glass or finished wood. Place the wooden frame on top of it. Center the frame, 3 inches of canvas outside each leg of the frame.
Wrap one side of the canvas over one leg of the frame and drive one staple through the canvas into the frame. Rotate the frame and painting 180 degrees. Pull the canvas tight, wrap it over the leg of the frame and secure it in place with one staple. Turn the painting 90 degrees. Wrap the canvas over the frame and staple it once. Turn the painting 180 degrees, stretch the canvas, wrap it over the frame and staple it once.
Staple the canvas to each end of the frame leg you are currently working with. Turn the painting 180 degrees, pull the canvas tight at the ends of the frame's leg and staple each end. Turn the painting 90 degrees, staple the canvas to the ends of the leg, rotate the canvas 180 degrees, pull the ends tight and staple each. Continue to rotate, stretch and staple the canvas to the frame until you have a staple along each leg of the frame, one every inch.