Just two things, in addition to the great info posted above:
When you're doing check-in ~45 days from sailing, you will be selecting a terminal arrival time, not a boarding time.
I've read on this board that if you're wanting a specific (early maybe?) arrival time, use the mobile app as it allows you to select your arrival time without going through all the prior check-in steps. Then when you have a bit more time, you can complete the rest of the check-in requirements either on the app or on your computer.