Turn the frame over and remove the metal clips if necessary. Lift the cardboard backing away to expose the glass or plastic. Put the picture or artwork into place and put the cardboard over the back of it.
Pick up one of the metal clips and pinch it together so the two ends touch. Insert the clip by slipping the pinched ends up under the back interior edge of the picture frame. Release it from the pinched position and it expands into place. It will expand so one prong of the clip is touching the cardboard and the other is touching the frame.
Repeat the process with a clip on the opposite side of the frame. Continue to install the clips until there is one on each of the metal frame's four sides. If there are additional clips, divide them evenly along the perimeter of the frame.