Simply wet your hair then wet the bar.
For shorter hair, you can lather the shampoo bar in your hands until you have enough lather, set the bar down, and apply the lather to your hair. Proceed to massage through your hair like you normally would. Rinse out the lather and proceed to conditioner bar.
For longer hair, make sure there is a sufficient amount of water in your hair to produce abundant lather. Apply the bar directly to the hair in downward strokes. Set the shampoo bar down and massage into hair like normal then rinse out, repeat if necessary and then proceed to conditioner bar.
Keep bars dry between uses.