No matter how you're traveling, it's always a good idea to change diapers and feed your baby, if needed, before you start out.
If you plan to breastfeed during a flight, inform flight attendants in advance. There should be no problem, but the close quarters can be awkward. The attendants might let you move to a less crowded area for privacy. For your own comfort, use pillows to prop up the baby while breastfeeding.
Some mothers use a waterproof airsickness bag with hot water provided by a flight attendant to thaw frozen breast milk in flight. If you're using powdered formula, bring bottled water to mix it. Don't use water from the tap on the plane because it might not be as clean.