Latex would work, so would a razor. The razor would be easier.
You would take the liquid latex and spread thin it on a smooth surface (like a cookie sheet).
Make two pieces this way. They need to be thicker in the middle and feather out so the edges are super thin, so you can blend them into the skin.
Glue them to someone's face
Then paint and blend them in with you girlfriend's makeup.
Thats the basics. Hope this helped.