Step 1: Fill a bucket with water and add some bleach. Than add the mixture in a vaporizer. (if you want to bleach the t-shirt first, unnecessary)
Step 2: Tape the lines of the form you want on your T-shirt.Step 3: Spray the bleach on you T-shirt.
Step 4: Start painting! Use a toothbrush to paint your T-shirt, make lines while brushing. Add some stars to make it look more cosmic.
Tadaa, your own galaxy T-shirt!