Sand the frame lightly to remove any foreign matter and smooth the surface. Brush off the frame to remove any sand paper dust. Spray paint black, let dry, repeat. When the frame is dry, use sand paper to distress it and spray on some clear glaze, I used a satin finish as I really didn't want it glossy.
Put the placemat on the back of the frame and trim it if necessary. Secure the placemat to the frame with hot glue, stretching it as you work to get a nice taut surface to work on.
Hot glue seashells and starfish randomly onto the surface in whatever fashion looks good to you.
I missed covering a stain, which I originally meant to cover with the largest shell, but after rearranging it never got covered. So I just used some jute string. I hot glue an end to the placemat, then just hot glued around in a spiral. I added a couple more to balance it out visually.
Next I needed to add a hanger. I was out of metal hangers, so instead I used some small nails and a piece of jute to create my own.