Spiral
fold
A common fold where fabric is pinched and twisted to create radiating lines.

To start a spiral tie-dyed shirt, you will pinch the center of the shirt and then start twisting. Then you decide how detailed you want all of the folds radiating from the center to be.
You can twist and loosely rubber band some structure and you'll be good to go. Or you can spend time pinching each accordion fold to be a precise, equal height all the way around the shirt.
It's going to be fun any way you go. That's what's great about tie-dye is you can decide how detailed and intricate you want to get with it.
Sometimes people will use a fork in the center of the shirt. They'll pinch it and then spin the fork while pleating the rest of the shirt evenly as they continue twisting the fork.