As it's Summer already, I thought I'd prepare this tutorial for you. Whites, naturals and shells add such an instant beachy feel to every corner of the house!
You will need shells...
A styrofoam ball, better if divided in half, so you can open it and then glue the two pieces together; a pot filled with pebbles and concrete (to give stability). The finished tree will be about 2 kgs/4.4 lbs heavy, so it needs a very solid base! Also, bare branches or cinnamon sticks (the ones I used here).
Assuming that you have already prepared your pot with a branch or cinnamon sticks in place, open the styrofoam ball, make a hole in it, and then insert the trunk and a few cardboard pieces. Glue them with hot glue. The ball will need to be well fixed, or you will have problems adding a lot of weight (shells) later on.
Then start attaching your shells, starting from the middle top of the ball. Start placing shells one close to another...
I believe it's better to fill all the gaps at this point, and finish off with tiny shells in the end, because you may want to see how your work prooceeds while it is in progress, and to mantain perfect roundness.
It would be useful to stand up from time to time and observe your tree from a distance! :)