BBT is based on temperature changes in your body just after ovulation occurs. You must learn from a professional nurse practitioner or doctor how to predict your fertile period, using changes in your cervical mucus and body temperature. Only after supervision of a competent instructor who can explain both methods. Don't have sex during fertile period unless you want to get pregnant. You could become pregnant unless you avoid sex during your fertile period.
Some women have trouble identifying mucus changes.
Many birth control clinics offer instruction in the Billings method. You can get a good thermometer at any drug store. You must have a very understanding, cooperative partner who will not insist upon making love during the period when you are fertile. Up to 2 weeks. Worn over penis during sex.