Here's A little step-by-step for you that uses the BAM stencil #1010, available in the BAM Complete Set. I actually did this as an entry in the Face Paint Forum's BAM stencil contest...you can check out the contest details here! The thought here was to create a mask that looks like a tiger peeking through the jungle foliage.
This was my first try entering one of their contests, and my first attempt at using this stencil. I made it up as I went along, and always have such a hard time painting on myself, backwards in the mirror, with my glasses off. (I'm SO super blind). But, I hope to come up with some better entries before the contest is up...you can enter up to 3 times! :-)