Remove the following 12 screws securing the front bezel to the rear case:

Eight 13 mm T8 Torx.

Four 25 mm T8 Torx.

On the 24" iMac Intel Model A1225, the bottom center two screws are long (26mm), four on sides (two left, two right) are medium (18mm), and the remaining 6 (four top, two bottom corners) are short (14mm).