First off, set the framerate to 30 and create an object on the stage that you wish to mask. For this example I have gone all out and drawn… a square. With a border. Oh yes, my artistic skills are truly blooming! Select the square, and press f8 to convert to symbol. Set it as a movie clip called MC_square and click ok. In the properties panel set the instance name to MC_square. Rename layer 1 to square.