We highly recommend checking in for your flights 24- hours prior to departure! You will need the 6-digit confirmation number or record locator which is the same. We also recommend you adding your cell number for text messages for airline schedule changes.
It gives you the opportunity to choose a seat, pay any applicable checked bag fees, and consider a variety of upcharges (such as an upgrade to premium economy or a fee to get bonus frequent flyer miles). You may also have to answer some security questions or supply a passport number for an international flight.
Once you’ve completed the check-in process, if flying domestically, you can print your boarding pass, have it emailed to you (so you can show it at the airport on your mobile device), or access it in a mobile app. If you don’t have access to a printer or a smartphone, you can print your boarding pass once you get to the airport as well.
Most airlines permit online check-in up to 24 hours before departure. The earlier you do so within that window, the better—because you’ll have more available seats to choose from. If you’re flying on Southwest, which doesn’t have assigned seating, early check-in is perhaps even more important so you get into an earlier boarding group and choose the seat you want.