How do i monitor my menstrual cycle to determine my best fertile days?

Calender : Keep a menstrual calender marking the first through last day of each cycle. Ovulation usually occurs 2 wks before the next scheduled bleed. Also, try otc home ovulation tests to see if/when you ovulate. Pregnancy is most likely if you have sex within the 5 days before ovulation and day of ovulation. Menstrual timing for sex only works if your cycles are regular/predictable. Cycles vary at times.