Cut off the flaps from the open end of the box using scissors, or have an adult remove them with a box cutter.
Paint the inside of the box black. You can use a brush and craft paint or spray paint; if you use spray paint, make sure to do it outside with an adult's help. Allow the paint to dry overnight.
Apply the star stickers to the black background, or hand paint stars with white paint. To paint stars, simply make tiny white dots all over the background. You can even create constellations.
Have an adult cut a 6-inch foam ball in half with a utility saw. Paint the ball half yellow, dry, and glue the flat side to one side of the box. Hold it in place until the glue has set enough that it doesn't fall off. If you're having trouble, stick a few straight pins through the outside of the box into the foam to hold it in place.
Draw each planet on a piece of poster board using your textbook or a scale picture of the planets as a guide. If you want the planets to be roughly in scale, start with Jupiter, the biggest planet. Otherwise, you may run out of room in your box.
Cut out each planet and paint it, using a picture as a guide. Paint both sides of each planet. Allow the planets to dry completely.
Tape a piece of string to each planet using clear packing tape. Tape the other end of the string to the top of the box, starting with Mercury closest to the foam sun, followed by Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and the dwarf planet, Pluto. If you want to add asteroids, place them between Mars and Jupiter, the location of the asteroid belt.
Trim away any hanging ends of string.