Keep a record or chart of the day you start your period and the day your period finishes.
When you do this for a few months, you will see that there is a pattern of when your periods start and finish.
By keeping a record of your period times, you will then be able to know when your menstrual cycle is due.
Some doctor surgeries have a monthly chart they can give you to fill in, or you can do your own chart or jot it down on your calendar or diary. Also write how you feel before your periods start. Some women get very emotional and feel irritable, have headaches, feel bloated, experience tummy or back pain and the nipples can be tender. This is called PMT (premenstrual tension) it is also know as PMS (premenstrual syndrome).
If you get feelings like that, a good healthy balanced diet with fruit and veg helps. And Evening Primrose Oil.
Hope that helps.
Best wishes.
You count from the first day of your last period to the first day of the next time you start your period.