Scalp serums like the ordinary should be applied after washing, on a clean scalp. Then how often depends on what you want and her scalp type. Scalp serums can be applied every day but you should monitor if your scalp is not becoming dirty too fast then. If your skin absorbs everything really well then it may be fine but if you notice build-up on your scalp after a few days of repetitive application you should consider using it less frequently. Please keep in mind what goal you’re trying to achieve and then monitor which frequency of application seems to give you the best results based on that specific goal (could be reducing itchiness, reducing hair loss oiliness for example).