I have made dozens of these seashell-embellished frames to give as gifts, and have even sold some of them.
It's an easy craft to do. You can use frames, shadow boxes, mirrors, boxes, - whatever captures your fancy. Each one takes an little as 20 minutes, or as much as several hours to make, depending on the size and how elaborate you'd like to make it.
Step 1: Materials
Plain wood frames also look good, but usually I cover these completely in shells, unless it's an otherwise dramatic or interesting frame.
You can purchase baskets or bags of shells at craft stores, or seaside gift shops. To buy a greater quantity, and spend less per shell, try online sources. I have had good luck with USshells.com.
The adhesive I prefer is Liquid Nails Clear Seal. It stays clear, and is gooey enough to keep the shells in place while I'm working on the piece, but wet enough to change things around if I need to. It may take a few days to be dry enough to package & give, so plan accordingly. I also like that when dry, there is still some flexibility to the shells in the piece. Sometimes a little give can keep it from breaking if someone hits it accidentally.