If you need to be really particular with a fussy cut, this method is a little bit longer but worth the effort.
Take a piece of paper and cut out the size square you need (say 5 inches). Cut carefully and remove the square in one piece so you have a "window" in your paper. Use the window to position over you fabric until you have the design visible that you want. Put the cut out square back into the hole, press your finger on it, and remove the outside paper leaving just the 5" square on the fabric to cut around. :-)