I started by buying a plastic ball at a toystore (note: I bought mine a bit too big, because I didn’t want to spend double the amount of money on it and I was a bit stressed with time).
Then I GENTLY sanded down the surface of it, so the paint would stick. More like scratching the surface than actually sanding it.
Afterwards I painted the entire surface twice with acryllic paint (not the one pictured, but something very similar). It did not cover fully, but since I was going to put crepe paper on it anyway I didn’t bother too much. If you’re going to be making something that isn’t entirely covered in crepe paper, I recommend maybe 3 or 4 coats depending on the quality of your paint.
I used the same paint as glue for the crepe paper and started from the top (!!) by ripping and laying on pieces of the paper. My initial plan was to use both white and this lime green paper, but I thought it looked better like this. Try to your taste! I went for a more anime/cartoon-y look, because I think this would look better in pictures from the convention :)