Before you get started: Because you'll be working with bleach, it is important to work in a well-ventilated area outside.
Wet bandanas. First, get the bandanas wet with water. Then wring out the excess water.
Add bleach into a bucket outside. Next, put on gloves and pour bleach directly into a large container or bucket (just enough to fully submerged the fabric). Again, make sure you're in a well ventilated area outside.
Dip in bleach briefly. Dip bandanas into the bleach and as they start to change color, remove from the bleach. This can happen pretty quickly (within 30 seconds).
Remove from bleach and rinse. Wring out the excess bleach from the bandanas and run them under cold water for 1-2 minutes.
Wash and dry. Wash in the washing machine and dry as normal.
Re-dye the bandanas. Once the bandanas have been washed and dried, they can be re-dyed new colors. I followed this dyeing tutorial.