Measure the piece of art you wish to frame. These measurements must include the length and width. Also, measure the depth of the piece, from the back to the farthest front point of the work. Measure the piece twice to be certain and write down the exact measurements of the work.
Go to a craft store to find a shadow box that fits these measurements. Unlike a traditional frame that covers a flat piece of art, a shadow box is a specialized frame that holds art with depth. Ensure the mixed media work fits inside the frame without extra space on either side, as you probably won't want to add matting to the piece. You might need to order a costume frame if one isn't available with your needed measurement.
Stain or paint the frame to match the mixed media artwork. The focus of the display should be the art, so don't choose a tint that takes away from the art piece. Instead, choose a hue that subtly complements the work it holds inside.
Remove the backing from the frame. If the mixed media artwork does not cover the entire back of the frame, decorate this board with a piece of paper or paint that matches the rest of the piece.
Attach the art piece to the backing of the frame. For lighter art pieces, use a double-sided tape to hold the piece against the board. For heavier pieces, apply a more secure method of attachment. Make holders that slip over the corners of the art piece and glue them to the backing with a strong adhesive, or string fishing line across the front of the object and attach it to the backing.
Place the backing, with the art attached, into the frame and secure it in place.