Score the film into strips with a utility knife
Peel up one corner with a razor blade or fingernail
Pull the film off slowly at a low angle
Spray the adhesive with soapy water, ammonia solution, or adhesive remover
Let the solution soak for a few minutes
Scrape off remaining adhesive with a razor blade
Reapply solution as needed to loosen stubborn spots
Wipe the glass clean with a microfiber cloth
Clean the window with glass cleaner or vinegar solution
Wear gloves and work carefully to avoid cuts