To create a “scratch” look using special effects makeup, take an angled brush like an eyeliner brush and coat with a cream-based red SFX makeup. Don’t add any alcohol to thin the makeup.
Ensure that the brush is nicely coated and then proceed to test it on your hand to remove some of the product. The makeup shouldn’t be coming off in globs.
Once you’ve tested on yourself, draw individual lines on the model’s skin using the thin brush. The number of lines will vary based on the effect you want to create.
Red on its own isn’t enough to create the depth that makes scratches look realistic. Add black to achieve depth. Black takes the scratch from simply being a red line on the skin to making it appear to be in the skin.
Using another very tight and fine brush, don’t add alcohol and test the product on your hand. Then, fill in the scratch. Begin about ¼ of the way into the red line you drew and stop about ¼ of the way to the end of the red line. (The red should extend beyond the black).
Finally, take thin, unmixed blood onto another thin brush and test on your hand. Come in over top of the black (ONLY over top of black, do not cover the initial red line).
Obviously, practice makes perfect, so go ahead and try a scratch out for yourself.