You don't pre-book a time. You head down to the shuttle area (close to DME, but off to the side). They will check you in and give you a character. So you generally decide when to head to the shuttle but then there may be a wait depending on how many people are there as it is first come/first served. We did this from the Hyatt a few weeks ago. We didn't rush to get the first shuttle but headed down closer to 10 am. We had to wait for the bus that was already in the process of loading and then another bus before our bus started loading.