You just need:
1. a dark tshirt
2. a spray bottle of bleach
Hang or lay the shirt out in a well ventillated area and spray it with bleach. Spray as little or as much as you want. It takes a minute or two for its effects to be visible. Everywhere the bleach touches will change color as most of us who have the unfortunate chore of laundry know well. The over all effect is pretty cool. You can add some cool colors to really bring out the galaxy look. Try folding or tying sections with rubber bands like in tie dye and see what happens.
When you are finished spraying, let the shirt sit for a few minutes, then rinse it out in the sink or tub to wash out excess bleach.
If you try this, please share a photo of how it turns out!
These shirts pictured were made by 6 year olds.
*****NOTE* I guess I should have looked on instructables before publishing this, there are several already out there and some much better than this one, oh well.****