You can select your seat on Allegiant Air by following these steps:

Book your flight: When you book your flight, you'll have the option to select your seat for an additional fee. You can do this online or through the Allegiant Air call center.

Manage your booking: If you've already booked your flight, you can log into your account on the Allegiant Air website and select your seat. You can also make changes to your flight or add optional services, such as a seat selection, baggage, or travel insurance.

At the airport: If you haven't selected a Allegiant Seat Selection in advance, you can do so when you check in for your flight at the airport. Allegiant Air's airport check-in desks open two hours before departure. However, seat availability is limited, so it's best to select your seat in advance to ensure you get the seat you want.

Note that Allegiant Air operates on a low-cost model, and they offer seating options at different prices, including premium seats, exit row seats, and seats with extra legroom.