Although I haven't personally done it, I understand that it can be done using something as simple as a large cloth funnel soaked in water. By setting up the funnel like a wind sock so it captures the wind at the large end and forces it to high speed through a smaller opening at the other end, evaporative cooling of the water will occur that will actually form ice near the smaller opening. The difficulty is most likely in the details, such as how often to re-wet the cloth, how large the openings should be for best efficiency, how much wind is necessary, making the ice at night to avoid heat from the sun, etc. Legend has it that ancient desert nomads sometimes made ice this way.