[edit] Steps

Make a list of spa supplies, such as food (should be healthy according to theme), and movies (ask the guests if they have a specific suggestion before the party)
Get invitations. There are some good ones at www.missoandfriends.com
Send the invitations. If it's a hassle to mail them, then find spa themed e-cards to send.
Decorate before the party with spa-themed deco.
Set out your food on a table.
Calm down and get ready for the best party ever.
When your friends arrive, tell them to unpack, because you shouldn't be wasting valuable time at the party.
Go outside for as long as you can because you're going to be cramped up for a whole night.
Make sure to have the girls bring a flashlight for late night bathroom breaks to flashlight tag
Eat your faces off, but supply extra snacks for the whole night.
Play games.
Do spa treatments the whole night, but while you do them, watch movies too.
Make sure you buy exotic face mask flavours like banana, pineapple, strawberry, kiwi,watermelon and oatmeal.

[edit] Tips

Make sure none of the treatments you use are bad for your or your friends' skin.
To keep the party alive, play truth or dare, pick totally diva movies, and have fun!
Make sure most of the food is healthy.
If you only have one other friend over give her beauty supplies!