You could go to your local Halloween store and buy bottles of liquid blood, or you could use things already in your makeup kit for DIY gore that’s free.
Shave off a bit of red lipstick, mix in a creamy black eyeliner, and finish with a dollop of thick, clear lip gloss. If you want brighter blood, add more lipstick; for a deeper, dried-blood hue, add in more liner. Then, mix together.
Apply the thick blood mixture using the end of your makeup brush, which is easier to wipe clean than the bristle side.
For a real American Psycho flourish, splatter the blood with a stiff brush like the Make Up For Ever Angled Shader Brush 234 ($30). Dip the brush into the “blood,” and run your fingers over the bristles to get a random spray of red.