This is not the most accurate method, but it's an easy, useful step to take while tracking other data. Purchase or create a calendar to begin keeping track of your monthly menstrual cycles. Circle the first day of your cycle, which is the day you start your period. Keep track of the duration of the cycle, which typically averages 28 days.  Record the number of days in each cycle, including the day you start your period. The last day of each cycle is the day before your next period starts. Keep tracking your cycle this way for eight to twelve cycles. The more cycles you have to reference, the more accurate the calendar method will be. Once you have at least eight cycles recorded, you can turn the data into a chart. List the day of the month your period began in one column, and the number of days in that cycle in a second column. Alternatively, you can use an online ovulation calculator such as the WebMD Ovulation Calculator. Make sure all of the information described below is included in the calculation, or it may be a less effective prediction. It’s difficult to pinpoint the exact day you’ll ovulate using only a calendar. Instead, a calendar is useful for predicting the range of days during which you will be fertile, which can be a different length for each woman, as described below:  Predict the first fertile day in your current cycle by finding the shortest cycle in your chart. Subtract eighteen from the total number of days in that cycle to find your first fertile day, meaning the first day in your cycle when pregnancy could occur. For example, if your shortest cycle lasted 26 days, your first fertile day would be the 8th day in each cycle (26 - 18 = 8), counting the day you began your period as day one. Predict the last fertile day in your current cycle by finding the longest cycle in your chart. Subtract eleven days from the total number of days to find the last fertile day in your cycle. For example, if your longest cycle lasted 31 days, your last potentially fertile day in each cycle would be the 20th...

TL;DR in Vietnamese:
Theo dõi chu kỳ kinh nguyệt song song với các phương pháp khác. Lập biểu đồ về số ngày trong mỗi chu kỳ. Sử dụng biểu đồ đã lập để dự đoán thời điểm thụ thai thích hợp trong chu kỳ hiện tại.