One of the many shirts I've made.
Simple mistakes to avoid:
Use all cotton, polyester won't absorb dye.
Wear rubber gloves, unless you want dyed hands for a week.
Rubber band shirts, soak in soda ash all night, wring out as much water as you can, dye, leave sit overnight, rinse then wash.
Lay down cardboard then a trash bag to dye on.
Wash tub immediately after rinsing out dye.
Seperate dyed shirts enough on trash bag to avoid dye running into other shirts.
Fun side project, get another white shirt and just ball it up, use it to wipe up excess dye during dye process. Instant chaotic and random extra tie dye shirt.