Step 3: Making the sky
This is where the circular objects come in. They need to be things that you can get spray paint all over. I went to a second hand store and bought some old pot lids, I also use the spray paint caps a lot, and pennies if I need really tinny moons.
Next, with the lids firmly in place, paint the whole paper black (or what ever color you want the sky). Make sure you get all around the lids, sometimes they block areas, and if you don't get it all it makes your planet look funny.
It's good to do this with a large piece of scrap paper behind your painting, or you will get it all over the surface behind your painting; like I did on the upper left corner (I forgot about that and need to clean that up tomorrow...)