Simply cut away the portion of the envelope that contains the clear plastic window.
Then open the portion you just cut away, and slide in a cut piece of magazine photo, your own photo, scraps of wallpaper, or whatever you prefer, and glue it to the inside, by placing glue on the back of your insert.
Glue your edges shut and allow it to dry completely.
Punch a hole at the top with a one-hole hand punch, tie on a scrap piece of string, ribbon, etc. and you have yourself a new bookmark.