To begin taking out the old return air filter grille, open the grille door and, if feasible, separate the door for safety reasons.
Unfasten the screws that hold the frame in place on the wall or ceiling, and carefully detach the frame. In case you are working on a ceiling installation, make sure to secure the frame/grille/filter to avoid it from falling and ensure your safety.
Measure the Duct Opening (the hole on the wall/ceiling) by measuring the WIDTH first, and then the HEIGHT (e.g 24" W x 12" H), this is the size you should order.