Most airlines used to provide these forms on the plane so you could fill it out prior to landing, but unfortunately most of them no longer provide this service. You will receive a form when you deplane, but you can also fill it out online (https://www.mexicotouristcard.com/) before your flight and jump ahead of everyone in the immigrations line. Depending on how long the line is, filling it out online beforehand could save you over an hour of waiting.
