If you have a large area and want to crop out or trial different areas, these simple frames are indispensable! They are simple to make from matt board or construction paper. You can make them in several sizes and you will definitely need several of them. Use an art knife and straight ruler for best results. Be sure to use black as it will further help to isolate the areas.
I've also heard of buying inexpensive matt frames and cutting them in half diagonally on two corners, how clever is that!