What you need:
Water from the tap
Add several drops of food coloring to balloon (the bigger the balloon the more drops you will need) . I had my son (we'll call him "G") hold the balloon and I added the drops.
Fill balloon to desired size. I put "G" on water patrol. He controlled how fast the water went in and when to stop the faucet. I held the balloon. Make sure you hold the balloons on the bottom because they get quite heavy and will pull off of the faucet and shoot colored water everywhere if you don't (not that this happened to us or anything....).
Tie balloon and place outside for 24 hours. I used bowls to hold up the balloons so we would get a uniform ball on the top of all of them. This left an odd shape on the bottom, but we are waiting to get a few inches of snow to bury them in. If you have larger bowls, it would probably work better, but we didn't have enough of the same size.
Once frozen, remove the balloons. Because water expands when it freezes, our balloons all split apart and were easy to pull off the ice. Each "marble" also had a crack in it, but as long as we were careful, they didn't break. (OK, one broke...)
Decorate your outdoors with colorful marbles! We had so much fun doing this, we made more today. A few to replace our broken marble and 5 more tiny ones.
I would recommend you be a "wee" bit smarter than me and wear gloves when adding the food coloring and filling up the balloons. My fingers look... colorful. :)