First, you have to buy or make some fake blood, of course. We trust you don't have plans to use the real thing!
There are dozens of recipes for making your own inexpensive version of fake blood. One ultra-simple recipe uses a tablespoon of water added to 3 tablespoons of corn syrup. That will serve as a good base. Then add red food coloring for bright, fresh-looking blood. For an aged or deep cut look, add a bit of blue. This type will work well on fake facial or arm cuts.