Apply a circular dab of 3D gel to the desired wound area. You will need to work quickly, because it will dry and become difficult to work with.
Use your cotton swab to create the texture you want for your bullet hole. Make it dip lower in the center of the gel to look like a hole.
Pour fake blood into your bullet hole. Let it run from the wound so it looks more realistic.
Use your cotton swab to control the flow of the blood and the way it runs from the wound.