You need a lipstick... not just any color, A brownish one would be perfect; an eye liner, a red nail polish and (optional) eye shadow. :D
First of all paint the area you want the wound on with lipstick. You don't have to follow any line or something, no need to be perfect... you wan a realistic wound after all.
Then draw some lines with eye liner, thicken them and most important do not use ruler! :D :P
After that apply red nail polish. Maybe you can use a darker one that would look more like blood. Apply it over the black lines where you think you should...
Allow it to dry. and if you want apply eye shadow on it. :D A dark brown or crimson would look great. ... Enjoy.