Kids love the power of being able to create optical illusions - and they're always shocked at how easy it is! I've done this lesson with students age 11 and up. Younger students make the shapes larger, and older students have fun overlapping, etc. Here is the lesson that inspired me, and another fun Op Art lesson is here too.
Step 1: Trace some circles on paper.
Step 2: Add curved lines (we call them horizontal "smiles and frowns" & vertical "c's and backward c's - OR "longitude and latitude" lines) to turn the circle into a sphere. Younger students can make less lines.
Step 3: Draw a grid for background, and remember to "jump over" the spheres with the grid lines.
Step 4: Color shapes, leaves every other shape white b/c the negative space is necessary to help create the optical illusion.
Step 5: Add shadows on sides of spheres.