Measure and mark the needed length of the frame on the outer edge of the framing piece. The inner edges will be slightly shorter as the cuts will be made diagonally inward from the top outer edge.
Attach the miter box to the end of your work table with a C-clamp at each end. Skip this step if you're using a table saw with a miter gauge.
Rotate the saw blade in the miter box or on the table saw so that it is set to make a 45-degree cut.
Place the wood into position, lining up the cut line with the saw blade. Clamp the wood down to prevent it from slipping.
Cut the wood. If you're making the cut manually, make the first cut by pulling the saw back toward you.