Using a screwdriver remove one end of the frame. There are many different types of frames and some may have tabs that hold frames together. These tabs need to be bent back to allow removal.
Place the old diffuser on top of the new Skypanels. Using a utility knife with a new blade, score the textured excess edges of the Skypanels 5 times with moderate pressure.
Line the scored side with the edge of a table. Working slowly down the scored edge, gently snap the plastic down to break off the excess, creating a clean finished edge. This should be an easy break. If not, re-score the Skypanels.