Decorating a child's room can be an inspiring and fun project for both the parents and the child. It will help make the room look better and will create the feeling of it being a sanctuary for the child. Painting stars on the walls is a fun way to brighten the walls and, if done right, will be really cool for the child.
Clean the wall with a clean cotton cloth and a general all-purpose cleaner to remove any dust, cobwebs or grime. If the wall looks good, there is no need to repaint it. Any color will work with glow-in-the-dark paint, but a white or off-white base will give better reflection.
Decide if you want to have a distinct constellation or just general stars over the wall. For a specific constellation, look up the star map on sites like: http://www.astro.uiuc.edu/~jkaler/sow/cm.html, and mark the wall with light-colored chalk. You will want to remove the chalk before you apply the paint.
Paint the largest stars with the green glow-in-the-dark paint. An effective way of doing this is by dipping your largest brush in the paint, working the paint into the brush and then pushing the brush hard against the wall so that the bristles push out in all directions. Pull back and push again a few times to work the paint out of the brush and onto the wall.
Design medium-sized stars with the same method, but using the blue glow-in-the-dark paint and a medium-sized brush. None of the stars should look the same, so don't be afraid of messing up. They key is to make sure the center of the star's surface is covered in paint.
Work the smaller stars into your wall sky by using your varying small brushes dipped in white glow-in-the-dark paint. These will be glowing a little dimmer than the rest of the stars.
Add variation to the starry sky by using the smallest brush to add little dots of the three kinds of glow-in-the-dark paint all over the wall. This will give the illusion of different depths.
Create a comet by cutting a kitchen sponge into a circle and then coating one side in the glow-in-the-dark green paint. Put it flat on the wall and then twist it in a circular motion, turn it slightly to the left and drag it away in a curving arch so that it leaves a trace of paint.