Basically each step in your form is a state in the state machine. When a user submits one step, you move the state machine to the next state. You can add conditionals for when to skip a step, and you can specify validations for each step that need to be run before a state-change (e.g. credit card needs to check out before moving from the credit card step to the confirm step).

You can check out Spree's state machine for the checkout process here: https://github.com/spree/spree/blob/master/core/app/models/o... (starts at line 83). Notice that the Order model is the state machine.