First purchase a digital BBT (Basal Body Temperature) thermometer, online is easiest.
Starting on day 1 of your cycle take your temperature by mouth in the morning before getting out of bed.
Take your temperature at the same time each day and mark it on the chart. If you take it more than 30 minutes earlier or later then note it at the top of the chart.
Use the chart to mark the days of your period by putting a cross in the appropriate box. Day one is the first day of proper bleeding.
Record any discharge through the cycle by marking the appropriate box.
Record anything unusual at the top of the chart which may affect your reading. An accurate BBT is dependent on at least 3 hours unbroken sleep before taking it. Illness, alcohol, air travel, and extreme stress may all alter the temperature reading.