After I made this, I searched to see if there were others and pikachu4592 made a really nice one that can be found here: https://www.instructables.com/id/Wearable-Shell-Mermaid-Crown/.
Step 1: Materials
Shells (I got mine at a thrift store but they can be bought at Michael's as well)
Hot glue gun
Optional: plastic pearls and fishing wire with a needle. This is in case you really want to go all out and put pearls all over the headband so you don't see the metal headband at all. I didn't chose to do this since the headband was hidden in my hair, plus I'm 6 feet tall so very few people will even see the top of my head.
Step 2: Sort the Shells and Design Your Crown
After I sorted them, I laid them out in the design and shape that I wanted.
Step 3: Create Crown Base
Step 4: Attach Shells to Base
I started at the end of the crown because I wanted the center of the crown to be on top of all the other shells. Think of a set of stairs and all the shells are stepping up to the landing of the center shells.
I bent the wire a little bit for every wire to make them fit nicely next to the shells.
Step 5: Embellish
I cut the leaf into three pieces and glued it to the back of shells. I then looked for any empty spaces or those I thought looked bare and hot glued smaller shells to those spots.