Step 2: Add some skin tone to the latex.
(BTW: test first for latex allergy if you have any doubts.)
It is important for sanitary reasons that you don't dip your brushes into product while it's in the bottle, but instead to dispense it onto a palette (or styrofoam or plastic plate). Cross-contamination can cause all sorts of yukky things to grow in your materials, and if you noticed, latex isn't exactly cheap (about $17 for a liter). This is even more important when it comes to greasepaint, and will be discussed in that step.
I have put a bit of runny blood in the lid here for the picture, but I would just allow a single drop to fall into the latex. Fake blood is worse than mustard in its ability to travel and stain, so use only what you need.
Stir the blood into the latex with the palette knife. *DON'T* use your brush; latex does not like to come out of nice brushes.
If you get it too dark like I did, pour out another drop of latex and use the mixture to tint it. It will dry a little darker, so err on the side of less tint.
All right: this is ethnically biased for pasty caucasians. If your skin is yellowy, you may be able to use just the latex without the tint. If you have darker skin, you'll need to use a little bit of ground eyeshadow or even a bit of instant coffee (mix with a drop of water first) to color the latex. While it's liquid, you can mix a water-based substance, including watercolors, into the latex. If you use anything oil-based, like foundation, it won't set up properly. But the runny blood will work for a lot of people, so try it and see how that goes.