Pour a drop or two of latex on the palette. Use a small bit of runny blood colouring to tint it so that it doesn't have that strange yellow latex colour when dry.
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 unwanted things to grow in your materials.
Stir the blood into the latex with the palette knife. *DON'T* use your brush; latex does not like to come out of brushes.
If you get it too dark, 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.
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. Do not use anything oil-based, like foundation, as it won't set up properly. Theatrical blood will work for most people, so try it and see how that goes