I don't remember the order, but here are the documents and information you will need for each passenger you are checking in:
Scan/photo of passport or (for US citizens on closed-loop US cruises) birth certificate -- I'm not sure if a photo ID is also needed for adults if a birth certificate is used
Photo (from the neck up, no obscuring elements such as hats, not the same as passport photos)
Emergency contact name and phone number
Payment information (such as credit card number)
Travel information (such as airline flight number, arrival, and departure times)
Once all of this information is entered, you will select a Port Arrival Time, which will also generate a Boarding Group number. If you have booked Disney transportation from a Walt Disney World resort or from the Hyatt at the Orlando airport, you will not need to select a PAT.
You will have the ability to indicate whether the emergency contact, payment information, and travel information are the same for all (or some) passengers -- if so, you'll only need to enter the shared information once.
You must upload clear copies of the identity document and photo in order to move to the next step. The site will say "approval pending," possibly for weeks, and if you need to re-load anything, you'll be notified.
If you are in a hurry to get your PAT, you can indicate "cash" for the payment information and "driving" for the travel information, even if that's not accurate, and go back later to update the information.