Check-in for your flight online if you have access to a computer and a printer. Point your web browser to Southwest Airlines "Online Check-in" feature and enter your flight and other identifying information to complete the process.
Click the "Print Boarding Passes" button, which will take you to a printable version of your boarding documents. Make sure your printer's inkjets are full, as faint and otherwise defective bar codes may not scan properly, potentially delaying your boarding.
Check-in at the airport at least 30 minutes in advance of your flight. Hand your identification card to the Southwest Airlines attendant, who will use it to look up your reservation. Obtain your multiple legs boarding passes from the attendant.