I know, I know: These are everywhere. Now to toss mine into the fray: ultra-simplified and with pics for every step.
Gather your materials.
Dismantle picture frame.
Use glass to trace shape onto plastic grid (I got mine in the embroidery section of the craft store). Cut out grid into final shape.
Hot glue the grid to the wooden part of the picture frame. Be messy, no one will ever see it!
Hot glue ribbon to the corners and let dry. Done!