I got the idea for these shell hair slides from Quiet Lion Creations.
Make sure to check out the original tutorial, especially since she went for really cool designs on her shells.
I went for a more natural look.
To make the shells into hair slides you fill them with Fimo/Sculpey. It's a chance to use up any odd bits you have because the clay won't be seen so it doesn't matter what colour it ends up as.
Then you press the hair slide into the Fimo, remove the slide, bake the clay still in the shell, then glue the slide back into the clay.
I painted my shells with nail varnish. One pair is silvery/pinky/redish.
And the other pair gradients from white to light blue.
I think the shells look really cute slotted into a messy up-do.