Because MastoAktiv is a food supplement in capsule form, the practical side is simple: follow the recommended use stated on the packaging. The product description does not specify a particular dosage or time of day, so the label is the only reliable source for the intended amount. The general way such supplements are used is to take the capsule regularly with some water, ideally at the same time each day so that it becomes part of a steady routine.
The product is described as fitting into a regular supplement schedule, and herbal products are usually assessed over a course of use rather than by one dose. If you are considering MastoAktiv for cycle-related changes, a reasonable approach is to give it at least a couple of menstrual cycles and keep simple notes on how you feel, so you can judge whether it suits your situation. It is not a quick fix, and no noticeable effect should be expected from the first capsule.
If you miss a dose, the common-sense approach that applies to supplements in general is to carry on with the next scheduled dose and not to take a double amount. MastoAktiv is not a substitute for a balanced diet; if your diet is irregular or you have specific nutritional needs, that is worth discussing with a doctor or pharmacist separately.