Measure your artwork and add 3mm to each dimension for a small reveal. Buy enough frame moulding (rabbet-routed profile) to go around twice plus 20cm for waste.
Set your mitre saw to 45°. Cut one end of a length at 45°, measure to the inner edge of your opening, and cut the other end in the opposite 45° direction. Cut all four pieces this way — two long, two short.
Dry-fit all four corners on a flat surface. The mitres should close perfectly with no gaps. Adjust cuts if needed.
Apply wood glue to all mitre faces. Assemble in a frame clamp or use corner clamps. Pre-drill and pin each corner with 25mm brad nails for extra strength. Wipe off excess glue.
Once dry, sand with 120 then 220 grit. Paint or stain to your choice. Cut a backing board and glass or acrylic to fit the rabbet. Secure with framers points or small turn buttons.