Select a brush shown above from the Brush Tool Menu, depending on the size of your document you may require a larger/smaller brush. Change the tolerance to 1px to keep the path close to the original mask.
With the Lasso Tool still selected, Right Click (CMD-Click) on the mask and select Make Work Path from the menu. Select an appropriate colour for your paper and fill the selection on a new layer.
Start with your blank document in Adobe Photoshop, use the Lasso Tool freehand to create the outline of the torn edge and continue the selection across the lower half of the document.
