When you are sure your parts fit together correctly, glue the frame sides together and clamp the frame around its outside edge to hold it. Be careful not to use too much glue, and use a damp cloth to wipe away any glue you see coming from the joins. For small frames you can use rubber bands or a belt type clamp. If you were making full size frames you might want to secure the back edges of the mitre joins with special framing nails called V nails available from art stores and framing suppliers. Make sure you apply pressure evenly to all sides of the frame to hold it as the glue dries.
The frame shown here still needs a lot of finishing before the picture is placed in the frame. It will need to be sanded (it has old finish on the wood). The rough mitre cuts (done with a power saw instead of a fine handsaw) will need to be filled with woodfiller, and the exterior and interior of the frame will need to be painted. As the frame needs to be finish sanded in any case after being glued, the original finish from this recycled piece of L mounding was not removed before the parts were cut.
Fixing the Canvas When a painting is ready to be fitted, the canvas is dropped into the frame from the front, and two small screws are screwed into the wooden stretcher bar at the top of the canvas to hold it to the frame. If your canvas is set onto a panel, you may prefer to use a linen artist's hinging tape (available from art supply stores to hold the panel in place on the top back and bottom edge of the frame. As the frame could possibly warp, you don't want to attach the canvas on all four sides or a warping frame might damage your painting. Hinging tape or double sided tape (archivally safe) can also be used to mount photos which are on foam panels to the upper back edge of the frame.
When your panel is mounted in place, you should finish the back of the frame with a layer of paper laid to cover the entire back of the frame and glued along the back edges. This will prevent dust from entering into the back of the canvas, although in floating frames, dust will enter into the side spaces.