This will be the same technique whether you are making a bead border, a shell border, or a ribbon border. The only difference is the decorating tip that you use, (straight for beads, star for shells, petal for ribbon).
It is exactly the same technique you just practiced with the teardrops
, only they will overlap each other. Make your first teardrop by squeezing with number 5 strength on the pressure scale, gradually reducing pressure as you pull back. Stop squeezing all together, and pull back to complete the tail. Now start the next bead by positioning your tip at the tail of the previous teardrop and continuing on as before, starting with #5 pressure and decreasing pressure as you pull back. Continue on for as long as you desire.
You may have seen a professional cake or cookie decorator make a shell border, and it appeared that they didn't stop between each shell, but most likely they did. They have just become so quick that it's hard to see. Practice doing it slowly at first and then work up your speed as you get more comfortable.