Filter tap or distilled water through a fine filter to remove any impurities from the water.
Boil the filtered water fully. Allow it to boil for two to three minutes to expel as much air as possible.
Pour the boiled, filtered water into ice cube trays. If you are using plastic trays, allow the water to cool slightly before putting it into the trays because freshly boiled water in plastic trays can cause them to crack in the freezer.
Place the ice cube trays into the freezer, preferably near the front. The key to clear ice is slow freezing to allow air bubbles to escape rather than become trapped under the ice.