While it may look complicated, cutting Hasselback potatoes is actually very simple. For the most fool-proof method, use pencils, chopsticks, or wooden spoon handles to place on either side of the potato lengthwise. Then, using a sharp knife, cut into the potato crosswise, around every ⅛th inch apart, cutting down until your knife touches the pencils/chopsticks (or leaving around ¼-inch if you have no 'guideline' object. Be careful not to slice the ends too thin, though, or they can fall off.