Yeah, it's so easy. You know the greek half man half goat guy Pan? And his flute thing? Well you sorta do it like that. What you gotta do is
1) pucker up like your gonna get muchas smoochas from Britney Speers, or whoever,

2) keep your upper lip out while

3)bringing in your bottom lip so that there is a small crevice between them.

4) then hold the shell or what not under your lips and gently blow.

For practice, you can also do this with soda bottles and shotgun shells. You can change the pitch by adding and subtacting water to it. More water is a higher pitch. Hope that helps.