I'm not sure if this will qualify as a good instructable, and even more so as its not any color of the rainbow, but here is a cheap and easy way to make seashell piercings from just thin thread and a thumbtack.
You will need:
1 string of thread
1 Index Card or Rubber-band
Step 1: Measuring the String
The string is measured out to be about 3.5 inches, then cut. Make it longer you want to dangle if you like!
Step 2: Tying the String
Take the string and pull one end through the hole on the shell. Tie it twice or more with knots to secure it.
(So the seashells I have were already pierced when I got them from the beach, making it easier to tie the string, If you're seashells are NOT pierced or have no hole, than try to find some that are please. Seashells are rather durable so piercing them neatly is going to be a hassle, believe me.)
Step 3: Make a Loop
Move up from the bottom knot you just made and leave a little space between them. Try to tie another knot above the bottom, twice again to secure it.
Step 4: Making a Back
Piece the end of the card with the thumbtack. Cut out around the hole, (make it small enough it cant be seen, but big enough it stops the thread from slipping).
This is simple enough. You can use anything, like rubber bands, paper, leaves, etc. I chose an index card since that's what I had at the time.
Step 5: Put It All Together
Put the thumbtack through the loop in the string. Cut off any excess string you might have from the knot. Place the paper back on the thumbtack.
Step 6: Done!
Voila! Feel free to paint or decorate the Flathead tack how ever you like and add some style! Now you can make use of those thousands of seashells you've collected :)