Rhonda

Turn the bookmark right side out and use a turning tool to poke out the corners.
Straighten it and iron it flat. I like to turn in the edges of the opening and iron as this helps to keep the opening straight when you sew it. I also put the bookmark under the needle and lower the needle. I use that to hold the bookmark and I gently pull against that to close the opening using my scissors or seam ripper to push the edges of the opening in and hold them in place if needed as I sew over the opening. Continue sewing around the edge til you get back to where you started. Back stitch.
This finishing seam will give your bookmark a really nice look. It will also take the place of any quilting in that it makes the fabric lay flat.