Mix cool water with wig shampoo in a sink or basin (never use a washing machine). Soak your wig for five minutes, or a little longer depending on how soiled it is. Even if your wig is quite soiled, never rub, twist or wring it.
After the five minutes, gently dip it up and down in the water. If there are makeup stains on the cap, clean them gently with a small brush (like a toothbrush).