If you know your ovulation, you can avoid unwanted pregnancy by using a condom or by avoiding sex during your fertile days.
To identify when you are ovulating, knowledge and experience are required.
Therefore, you need to use a fertility app or take a course.
After ovulation, a woman's body temperature rises, and when this rise has occurred, you know that ovulation has taken place. At this point, you can have unprotected sex without becoming pregnant.
A fertility app tracks this temperature rise. For example, the company Natural Cycles does this, and they are well-established in the market. We advise against using a regular period app or tracking it yourself, as there is a risk you may not be adequately protected.