While these are not glass balls, as was recommended, these are SAFE balls... AKA plastic. This was so easy and so much fun that I wish we had more than 8 balls to paint!
You take Mod Podge (because it dries clear) and pour a tablespoon or so (didnt measure) into the bulb, then put paint into the hole. Try not to put them in the same place. These bulbs are huge, smaller ones might be hard. Then roll the ball slowly around in your hands until it is completely coated. I had to add more paint a few times, but this was my first try. The Mod Podge aids in the marbling. I also bought the one that said "Pearl" on it, so these are extra pearly marbling.
Funny story: We got home from school and my son was busy doing something really quietly while I was doing 2 million things. I came back into the room. I had not even told my 5 year old how to make one. He had poured 3 different colors into a bulb and he had even put glitter into it. I was quite impressed, but he needed the Mod Podge to make it coat and marble correctly. I added the Mod Podge and BAM! It is beautiful too!
Make sure to google "Michael's Coupons" or "Joann's Coupons" to find a 40% off coupon (or better) which can be used at either store.