Because the mask is made for both the scalp and the lengths, it works well as a step after shampooing. Wash your hair with your usual shampoo, squeeze out some water, and apply the mask to damp hair. Work it into the roots and scalp so the ingredients reach the areas where oiliness and flaking show up, then spread it through the lengths as well. If the label gives a leave-in time, follow it; if not, a few minutes before rinsing is a practical starting point. Rinse thoroughly so no residue is left behind.
There is no fixed frequency that works for everyone. The 946 ml format suggests the mask is intended for regular use at home, including for people who wash their hair often. A reasonable way to begin is once or twice a week and then adjust based on how your scalp and hair respond. If the hair starts to look heavy or greasy, you are likely leaving too much product in the hair, either by using too much, not rinsing well enough, or using the mask more often than your scalp needs.
This type of product is more active than a plain conditioner, so the product notes advise paying attention to skin sensitivity. If your scalp is very reactive, irritated or damaged, introduce the mask gently. Stop use if discomfort appears and avoid contact with the eyes.