To book a multi-city United Airlines flight, you can visit their website and use their booking tool for flights with multiple destinations. When using United's multi-city booking tool, you can search for flights with up to four destinations by inputting each city and your travel dates. Additionally, you can choose different class options and add or remove additional booking preferences such as flight type, preferred airline, or price range. United Airlines Multi City offers numerous destinations to choose from and sometimes features exclusive vacation packages. You can also earn miles by using the MileagePlus program through United Airlines which can be redeemed for future flights and other travel-related awards.