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.
Step 2: Attaching Shells
Select some shells, and play with them until you like the arrangement on your frame. Just do a few at a time. With a toothpick, get a gob of adhesive, and put it on the shell where the shell will make contact with the frame. Press the shell onto the frame. Glue a few shells on, then look at it, and repeat the process.
If covering an entire frame, I first put a 'base coat' of plain, fairly flat shells like scallop shells. This gives a shell background, so that if there are spaces between your shells, you have shell showing through them, not the frame itself.
Put larger shells on first, then smaller and smaller shells. I almost always finish with some really tiny shells (about 1/4 inch), which fill in the inevitable gaps nicely.
When you're satisfied with your work, let it rest horizontally for a couple of days, until completely set.
If you get glue on the glass or frame accidentally, it helps to clean it off right away. Take the bulk of it off with a toothpick, then wet a q-tip and rub till the spot is gone. It will come off when dry, but I have found it easier to take off when wet.
Step 3: Experiment With a Variety of Frames
The simplest frame here has less than 20 shells. The largest - the oval mirror, about 22 inches in length - has over 300.
Here are pictures of some other frames and mirrors I've made, and mostly given as gifts.
Have fun, and Happy Holidays!