Slice the lemons and put one round slice into each compartment of a muffin tray. USA muffin pans work well for this since they are coated with silicone. (You can also use regular ice cube trays as well. If you do, you'll need to cut the lemon slices into fourths.)

Fill the cups with water and freeze.

Once they are frozen, pop the ice pucks out of the tray. Run a little warm water on the bottoms to loosen the "cubes" if they're stubborn.

Store the frozen slices in a freezer bag in the freezer.