Start by slicing the lemons in half, and removing as many seeds as you can. Then either slice them thinly by hand, or use the food processor, with a 4mm slicing blade (the 2mm makes slices so thin you can see through them. Nice for garnish, but not so good for the freezing process). Lay them in a single layer on the cookie sheets, freeze until hard, then remove them from the cookie sheets and put them in bags and put them back in the freezer.
You can use them in water or drinks straight from the freezer; they act a little like an ice cube as they thaw out and flavor the liquid.