I just used a blank frame and glued (Mod Podge) on the sand. I used store bought sand for this project. You can use beach sand, just make sure you sift all of the larger stones and other debris that may be present. After that dried, I covered the sand with another layer of Mod Podge. I then pressed the shells into the sand, and used a toothpick to write/scratch 'Puerto Rico' at the bottom.
You can also use this same idea with a candle and beach glass by layering sand + glue and pressing glass pieces into the sand so they are touching the candle. This way, the light comes through the colored glass and creates a really cool effect.