Blarg

• Make a layer with whatever it is that you want to mask. Like text or an object, like a circle.
• Create another layer with an object that will work as the mask. Such as a square.
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• Go to modify on the menu bar, timeline and lastly, layer properties. Make the layer options into a mask for the second layer so that it hides the object on the first layer.
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• Go back to the first layer, layer properties and set it to masked. 
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• Sit back, and watch how your movie is masked, like a burglar at night.