Talk with your physician to ensure that it is safe for you to fly. Make sure that the boot for your ankle is loose enough to remove while in the air. Flying causes swelling in the extremities, so it is important to have enough room in the boot for your ankle and leg to swell without cutting off circulation.
Call the airline you are flying with to find out if they have any policies regarding flying with an injury. The airline may require a wheelchair to take you through the terminal before and after your flight. The airline representative can also look up your seat on the plane to determine if you will have enough leg room.
Wear a compressive sock or bandages under the boot if it is removable. This will help reduce the swelling that occurs while flying.
Ask about boarding the plane before other passengers to avoid further injury from being in a crowded space. Take your time while moving around and keep your foot out of the path of the aisle.