Radical Enzymatic Scalp Exfoliator is a pre-shampoo scalp treatment, so it should be used before your regular wash. Apply a small amount directly to the scalp, either on damp or dry skin, working through the hair in partings so the product reaches the roots rather than sitting mainly on the hair lengths. You do not need to coat the whole hair shaft; the product is intended for the skin under the hair.
Leave it on for the time indicated on the packaging, then wash your hair as usual with shampoo. The shampoo step is important because the exfoliator loosens build-up such as dead skin cells, sebum, sweat and styling residue; washing afterwards helps rinse this away. If you use conditioner or a hair mask, apply those after shampooing, mainly to the lengths unless the product instructions say otherwise.
A typical frequency is one to two times per week. More frequent use is rarely needed and may make the scalp feel overworked, especially if your skin is sensitive. If you are trying it for the first time, starting once weekly is a practical approach. Pay attention to how your scalp feels afterwards: a clean, lighter root area is expected, while burning, persistent stinging or visible irritation means you should rinse thoroughly and stop using it.