Cooling towels (or evaporative towels) are specially designed towels intended to be soaked in water and placed on your skin. They provide significant cooling effect as the water evaporates. This can be an effective way of keeping cool on hot days, though they are more effective in drier climates.
To use a cooling towel, just get it soaking wet and then squeeze most of the water out of it. Then, place it around your neck or on other areas of your body. When the towel has dried out or warmed up, repeat the process.
When you are done using the towel, rinse it out to remove any sweat it may have absorbed. You may roll it up for storage. When it dries completely, it will be stiff. When it gets wet again, it will be pliable.
If you don't have a cooling towel, regular wet towels also work okay, but they tend to not hold the water as well and you will get wetter. They also may not stay in contact with your skin as well, which is necessary for the cooling effect to work. Many other cloth materials will work, including rags and wet clothing
For more cooling power, if you have a refrigerator or cooler, chill your damp towel first inside a plastic bag or similar container, and then put the cloth on your skin until it warms up.