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Choose a large, scalloped shell, starfish, or sand dollar as your focal point. Glue it down in the center of the crown with hot glue or industrial strength glue. Pick out your pointy shells, and glue then along the edges of your crown. Make sure that the opening of the shell is facing the tiara's band; this way, the hole won't be visible in the end. For a more interesting effect, glue the taller shells towards the center of the tiara, and the shorter ones towards the ends.  You can add as many spikes as you want. You can have them go from the focal point all the way to the combs, or you can have just a few on each side. For a more interesting look, alternate between turret and conch shells. At this point, the base of your crown is done. You can make it look more interesting by adding more layers. For example, you could glue some clam or snail shells along the base of your crown. A smaller clam shell to either side of your focal point would also look pretty. If you used a sand dollar as your focal point, consider gluing down a tiny starfish in the middle. You may have some gaps between your shells, especially along the bottom. You can fill these in with small items, such as glass beads, pearl beads, or tiny rhinestones. Bits of sea glass would also look pretty. Your crown is essentially done at this point, but if it feels like it's missing something, consider adding some rhinestones. Use super glue to attach tiny rhinestones to the shells. You can put the rhinestones around the edges of the shells or down the ridges. If you used a starfish, you could add rhinestones down the middle of each arm.

Summary:
Glue down the focal shell. Add some spikes along the sides. Create more layers. Fill in the gaps with fillers, if desired. Finish off with the details.