Take a shirt.
Tape it up.
Oh, and stuff it with paper.
Spray it with a spray bottle of watered down fabric paint (it just has to be watered down enough to be squirtable).
Use several colours of paint, if you wish.
Repeat with the other side. You can either wait for the first side to dry, or just go balls deep and do everything all at once; it’s messier if you do it all at once, but it’s also faster. Your preference.
Wait for it to dry, then peel off the tape.
Another technique you might be interested in is the Pollock-Paint-Pour, where you drip and splatter the paint onto the shirt instead of spraying.
- You could probably attempt a more artful Pollock-Paint-Pour technique than what I did here. Yes, all I did was dump the leftover contents of my spray bottle. And yes, it all came out in one big blob.
After the shirt is dry, you should heat-set the paint. Turn the shirt inside out, place a towel on it and iron for 3-4 minutes. Then wash it to remove the excess paint.