Wash your denim item with only a teaspoon of detergent and no fabric softener. Allow it to air dry.
Using a wide brush, apply a thin coat of rubber cement to the area of denim that you will be painting. Allow the denim to dry, and apply one more thin coat to the area. This will provide a good base for the ink, much like a primed canvas.
Using a round artist's brush, apply a thin coat of silver paint to any areas where you plan to use bright colors like yellow and red. This will make them pop and prevent them from sinking into the denim canvas.
Once all base coats are dry, use a nylon bristle brush to apply the inks to the denim. Nylon brushes are stiff and help the ink to adhere to irregular surfaces like denim. They may be purchased at any craft or art supply store.
If you make a mistake in the application process, wipe it away using a damp paper towel or sponge as soon as possible. This, along with the base applications, will prevent the inks from setting quickly.