Clean helmet to make sure it is grime and dust-free.
Optional: Tape out the inside of your helmet. You don't want to get paint inside on the foam.
Use a drawing transfer technique or sketch your design on your helmet with pencil.
Paint the helmet with acrylic paint.
Allow paint to fully dry.
Use a foam brush to apply one even layer of Mod Podge on the outside shell. Be careful to not get any inside the helmet.
Let first layer fully dry.
Paint a second layer of Mod Podge.
You can add a third if you want. Let helmet dry overnight.