All You Need:
A white shirt
Acrylic paints and spray bottles (we used squeeze bottles)
What You Do:
In your bottles, mix you paint in your water until there are no chunks left. The more paint the darker the colours will be.
Wet your shirts and then squeeze most of the water out of it.
Tie your shirts whatever way you want to get whatever kind of pattern you want, securing with elastic bands. We found that the best way was the traditional spiral-style, which gave better coverage and design. (Spiral tying: http://www.youtube.com/watch?v=2PJnM6MK30U)
Then, let your kids spray or squirt their shirts with the paint!
We left ours to dry tied up for a bit, but we became impatient and unfurled them to let them dry on a clothes line.