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.
Step 2: Mix Paint With Water
Step 3: Wrap the Shirt
Start wrapping the string in a spiral fashion up the shirt, leaving an inch or two between the wraps. The wider apart your wraps are, the thicker your stripes will be. The closer together your wraps are, the thinner your stripes will be.
When you run out of room to wrap the string, slide the part you've already wrapped down the bucket (towards the hem), and then keep wrapping. The shirt will wrinkle and the wraps will overlap each other, but that's how you want it to look.
When you get to the sleeves, just try to flatten them out as best you can. Keep wrapping the string and sliding the wrapped portion of the shirt down until you have no more shirt left to wrap. Tie the string off to hold everything in place.
The wrinkles should be fairly even, but don't stress out about trying to make it perfect. It'll look great when you're done!
Step 4: Paint It!
Spray the shirt evenly, all the way around. Every part that you can see should be the color of the paint, when you're finished.