Check the airline for availability for the flight you want to take by calling it directly or searching for day-of flights on its website. Trying to fly standby on an earlier flight can be more reliable, since you'll still have your confirmed reservation until you board the earlier flight.
Arrive at the airport at least two hours before the flight you would like to take departs. It behooves you to take only a carry-on, since standby passengers without checked baggage will be considered first.
Ask the ticket agent if you can check in as a standby passenger, if they have not opened your original flight for check-in.
Proceed through security and to the gate where your standby flight will take off.
Ask the gate agent to put you on the list for standby, if the ticket agent has not already done so. If you want an immediate seat confirmation on the standby flight, airlines will generally charge a fee between $50 and $75. Hold out for a last-minute seat assignment to avoid that fee.
Wait for the agent to call your name after the confirmed passengers have boarded; if there is no seat, you will be put on standby to take the next flight.
