Determine size of frame by measuring the outer dimensions of the photograph, painting or mat. These will be the inner dimensions of the frame.
Choose the type and style of wood molding. Determine how much molding you need by adding the length of all four sides of the mat together and then adding the width of the molding eight times (twice for each corner).
Cut the wood into four pieces using a manual or power miter saw. Cut the pieces at 45-degree angles.
Lay the wood with the back of the frame down, so that the saw begins cutting on the face of the molding. This will help prevent splitting on the front of the frame.
Be sure to cut the molding so that the short side of the frame matches the length of the mat---it is easy to accidentally cut the frame too short by matching the long side to the mat.
Assemble the frame using wood glue and clamps, attaching the sides with staples, finishing nails or v-nails.