There are special shampoos that can be used for synthetic fiber. They will add moisture back into the fiber to give it a natural appearance and will help to reduce static.
Brush the hair thoroughly before washing. Fill the basin with cool water. Add a small amount of shampoo to the water and swish around. Place the hair into the basin of water and leave for several minutes. Turn the inside of the base out and clean the inside of the cap with a little more shampoo.
It is important to remove the oils from your scalp that might build up on the cap. Rinse thoroughly and repeat the shampoo process if needed. You may apply some conditioner designed for synthetic fiber if needed. Be sure to rinse well. Gently squeeze out the excess water. Shake out any water left in the hair and place on a folding stand or over a tall bottle so that air can circulate under the cap. Let the hair air dry.
For best results, do not brush the hair until it has dried. It will fall back into the same style that has been set into the synthetic fiber. It is very important to never use hot water, a hot blow dryer, or any type of heating element on the synthetic fiber.