Because of the way black soap is made and because of its unique texture it is best to use just pieces of the soap instead of the whole bar in the shower or bath. Simply break off or cut pieces from the soap as needed.
Black soap should never be applied to the skin directly. Remember, this is a very, very natural soap and some of the ash used in the soap making process can still contain large pieces of plantain which could make it a bit scratchy or irritating. Instead, make a lather with the soap by wetting your hands and rubbing the soap between them. Then use this lather to wash with. Make sure to rinse off thoroughly. Another way to use black soap is to make a lather on a wash cloth, sponge or loofa and wash with these as you normally would.
Black soap also makes an excellent shampoo for your hair. To use it as such, make a lather with the soap with your hands then work the lather in all the way to your scalp then rinse thoroughly. Never apply the actual soap directly to your hair as it can cause the ends to split or break. Follow up with your favorite moisturizer or conditioner if you prefer.
Remember to always store your soap in a dry place. Black soap absorbs water quickly and will not last very long if it is allowed to become soggy by leaving it in the shower or bath.