To claim past miles on Frontier Airlines, visit their official website and find the "Claim Missing Miles" section. Fill out the form with accurate details, including your name, frequent flyer number, flight information (date, flight number, origin-destination), and attach any supporting documents like boarding passes. Submit the form, and the customer service team will review your request. If approved, the missing miles will be credited to your account. Keep copies of your documentation for future reference. File your claim as soon as possible after the flight for a better chance of success.