This is difficult to do with an ice cube tray. Cubes often appear cloudy because of impurities (generally unharmful) in your water. As the water freezes, the impurities are pushed toward the bottom and center of the cube, where the water is still in liquid form. As a result, the only way to get clear cubes is to freeze a large amount of water and chop cubes out of the top of the ice where no impurities have collected.