The secret is to get the ice to freeze in one direction — from to to bottom. Freezing the ice in a cooler, with the top open, will make this happen. The insulated sides of the cooler keep the water from freezing on the sides or bottom.
Keep the cooler in the freezer for twelve to twenty four hours. Any longer, and it will begin to get cloudy on the bottom.
Once the water is frozen, you just wedge the clear ice into appropriate sizes. There’s a good instructional video below: