To get started, grab a few jumbo craft sticks and a pencil. You can freehand the characters, or use the pattern provided here.
You'll want to paint certain areas first on each superhero. For Iron Man, paint the gold sections, on The Hulk paint the green, and on Captain American paint the peach and the white section where the red & white stripes go.
After those have dried, paint the purple on The Hulk and the black for his hair. Paint the red stripes on Captain America, and paint the red section and the red helmet piece on Iron Man. When the stripes on Captain America have dried, paint the blue areas.
Once all of that dries, use a white paint pen to add the following:
The Hulk - mouth, and eyes
Iron Man - Unibeam (circle) and eyes
Captain America - Star, "A" on helmet and eyes
Use a black Sharpie to add The Hulk's eyebrows and pupils. A ballpoint pen works well for the mouth outline and the teeth. A black Sharpie can be used to add Iron Man's mouth and Captain America's pupils.
Let everything dry well. Add a coat of paintable sealer or Mod Podge to give them a protective coating. Let dry overnight before using in a book.