One of the most convenient ways to check in for your Spirit Airlines flight is through online check-in. This option is available starting 24 hours before your flight's scheduled departure time. To Spirit Airlines check in online, simply visit the Spirit Airlines website, enter your confirmation code and last name, and follow the prompts to complete your check-in. You can choose your seat, purchase any additional baggage or other optional services, and print out your boarding pass or save it to your mobile device.