First of all you want to wash the shells dipping them in water with a teaspoon of bleach and rub them with a brush to take off all incrustations.
this will avoid the shells to stink..

Now choose the thread and lay out the shells onto the bikini the way you want them to look...

sew the shells along the bikini edge and fix them with a knot on the back

Go all the way down and you're done ;)