In the Tesla app on your phone. One of the pre-delivery tasks is Final Payment or something like that. Once you're close enough to delivery, the option to do it will actually be turned on. It walks you through the process but basically you temporarily log in to your bank account through the app, via routing and account numbers and a one-time passcode, and transfer the money to Tesla right then and there. It's quite slick, and a lot easier than having to go to the bank to get a cashier's check for an exact amount.