Gather your materials and stretch them out on a clean table. Ruler, pencil, white plastic erasure, acid free tape, hot glue gun, white matt board that matches or compliments the artwork, acid free foam core pieces cut to slightly smaller than the artwork.
I replaced the original backing with sturdy matt board making sure it was larger than the opening so that it would attach across the back of the frame. Measure and place the paintings keeping an equal space at the top, bottom and sides. paces between the paintings should be slightly less. I left 1 1/4 inches around the edges. I put very light pencil marks in the bottom corners only. Put coiled tape on the back of the foam core squares and attach to the matt board. After they are mounted and the measurements look good erase your pencil marks. Now let's put it all together, hot glue the plexi glass to the inside of the frame. Attach the matt board to the back of the frame, before securing it in place check for dust, debree and fingerprints within the frame. Attach the matt board to the back of the frame, I used the existing clips and a staple gun for the side without clips.
For light frames I attach fishing line with a staple gun and knot it in place for hanging.