To select a seat on a Lufthansa flight, log in to your booking on the Lufthansa website or mobile app. Navigate to the "Manage Booking" section and enter your details. Once your flight is displayed, you can choose your preferred seat from the available options. If you prefer, you can also select a seat during the booking process or at check-in. Note that some seats may incur additional fees. For assistance, you can contact Lufthansa's customer service at 1-860-516-6375.