What program are you using?
If it's Photoshop or Elements, you can use the shape tool or the cookie cutter tool. If you use the shape tool, it's nestled with the pencil tool and a few others, make a shape by choosing one from the drop down menu bar across the top of your work space. Double click on the shape, then close the shape picker, click on your document and draw out with your mouse. Then you can drag your paper on top and clip it to the shape with Control + G.
Is that what you meant?