You'll want to start with a white or light-colored shirt and darker-colored fabric paint. It's much easier to use dark paint to cover up a light fabric, than to use light paint to cover a dark fabric. Also, avoid any fabric paints that have glitter in them. The glitter will clog your spray bottle! You can use only one color, like I have done, or let the first color dry and start the whole process all over again to add a second or even third color to your shirt.
Also, you might be thinking, "Hey! I'll just get a can of spray paint from the hardware store!" Well... you can, but the shirt will stiffen everywhere the paint touches it. It won't be nearly as pleasant to wear, as it will if you get paint that's made specifically for fabric. Most craft stores have it, and it's not expensive. So, I'd recommend that you spring for the good stuff!
Step 1: Gather Materials
A light-colored or white t-shirt
Fabric paint that complements your shirt's color
A spray bottle that sprays a fine mist
A glass marble
A bucket, pipe, or pot that is about as big around as your shirt's collar (Be warned; you will get paint on it!)
String, yarn, or heavy thread
The shirt I chose was kind of a dull yellowish-green. I wanted to liven it up with some brighter green tiger stripes.