Remove the glass from your frame. Place it on a flat piece of thin cardboard and trace it. I recommend using cereal box cardboard. If you use something too thick, you risk your finished product not fitting into the frame.
Wash the glass using dish soap or glass cleaner. Set it aside to air dry.
Cut out the traced shape on the cardboard, cutting completely inside the lines. Your cardboard should fit inside the frame (where the glass will rest) with a few millimeters of wiggle room on all sides. Err on the side of more wiggle room, not less.