Great question. Your basal body temperature (BBT) is your body's temperature at rest before you get up in the morning. Because temperature can fluctuate throughout the day, taking your temperature first thing in the morning - before you get up, or eat or drink anything - gives you the most accurate reading.
Keep a thermometer by your bed. (You can use a digital thermometer or an old fashioned glass one with 1 degree separations - it's just easier to see.)
Start around the time of your period, taking your temperature every morning when you wake up - before you get out of bed, and before you put anything in your mouth.
Mark your temperature each day on the ovulation chart and then connect the dots.
What you're looking for:
Before a woman ovulates (produces an egg), her temperature will be low. After ovulation, her temperature will be higher. So, ovulation will occur sometime during the days when the temperature moves from low to high. It's best to time sex for when your temperature begins to increase.
Sometimes, health care providers like to see your recordings because it can help us determine easily when you’re ovulating - without you spending a lot of time and money on lab tests and ultrasounds. It also helps us advise you when to have intercourse.
What is my basal body temperature? 
