Frost your cupcake with white frosting as a nice contrast to the black gel
Use the black decorating gel to make a target on your cupcake with a dot in the center and two circles around the center dot with space in between the circles and the center dot.
Take a toothpick and drag it from the center dot to the outside of the circle, going all the way through the outside circle to the edge of the cupcake. You'll do this evenly spaced around the cupcake, making eight different lines.
Use a paper towel to wipe toothpick clean when it is needed