Print out the template I have attached, or draw your own.
Cut out parts of the shapes to make stencils.
Fold a sheet of newspaper into thirds or fourths. This will go inside your tights/leggings.
If you want, try on your tights and mark the knees with tailor's chalk so you know where to position your stencils. Lay out some newspaper for your workspace. Then, put the newspaper you folded into the legs of the tights so that they lie flat.
Choose a moon phase and lay the stencil over your tights. Dip the sponge in the paint and dab the paint on the tights; this will create a realistic moon-like appearance.
Using the same stencil, repeat on the other leg.
Repeat steps 5-6 with the other moon phase stencils.
When you are done, let it dry completely. Then, you can take the newspaper out. :)