Keeping a record of your menstrual cycle is absolutely essential because many rulings depend on it. It is extremely important to keep a record of the exact time and date when bleeding begins and stops. In cases where habits become irregular and problems arise, it is keeping a record which saves a woman from unnecessary hardship of rulings such as having to perform a Ghusl for every Salaah.
A simple calendar can be quite sufficient though nowadays it is quite easy to keep a record with various apps which are easily available. Attached are different types of charts you can print out.
Edit: I cannot seem to be able to attach the charts