Purchase some wood. Cedar makes the best canvas frames because it doesn't warp. If you are going to be making several frames, talk to the manager of your local lumberyard and see if he will give you a bulk discount.
Cut the wood. The length of wood you need is determined by how big a project you anticipate. For example, an 8-by-6 inch canvas will require two 8-inch pieces of wood and two 6-inch pieces.
Use a table saw to cut both ends of each piece of wood at a 45-degree angle.
Use hot glue to fasten all of the corners together. Make sure the corners are tight by placing them in corner brackets and allowing the glue to dry overnight.
Stretch the canvas tightly across the frame and staple it in place. Make sure the staples are in the back of the frame, where they can't be seen.