If you already have a ticket, you must show up at least 30 minutes before the scheduled departure for the short check-in process. If you’re late, they might sell your slot to chance passengers. Once your tickets have been resold, you will no longer be allowed to board.
If you booked online, give yourself an hour or two before the check-in time to claim your ticket. You will be emailed a confirmation that you need to print out and present at the ticketing booth to get your actual ticket. They have the right to forfeit your reservation and resell your slot if you’re late.