Tint a can of frosting red or make your own homemade icing. Ice the top of your cupcakes with the red frosting. If you are picking up your cupcakes from the bakery, you can pick them up with no icing or have them add just plain red. NOTE: Gel food coloring gives you the truest red color. I would recommend it over liquid.
Add blue icing to a piping bag with a small tip. Make three circles on the top of each cupcake. One in the very center, the second in the middle, and the third close to the outside edge.
Use a knife and run it from the center to the outer edge in several places around your cupcake. You are looking for a spiderweb effect.
Then just add a spider ring to the center of each cupcake by pressing it down in the center.