1. old tee
2. bottle of bleach - squirt all over the shirt in desired areas
3. soak in a bowl of cool water with a small amount of laundry detergent (this helps sets the bleach so that it won't fade or bleed when you wash it later with other articles of clothing and it also helps not having it stank of bleach for days.)
4. rinse well and let dry
Easy DIY for a quick, casual Summer outfit.