For (half) a year you make a note of the date your period starts.
After half a year you calculate the shortest cycle (the least amount of days in between periods) and the longest cycle (the biggest amount of days between periods).
Let’s take Barbara as an example. The past 6 months she wrote down: 28 days, 30 days, 29 days, 32 days, 31 days and 29 days. The shortest cycle was 28 days, while the longest was 32 days.
From your shortest cycle you subtract 18 and from your longest 11.
With Barbara that comes to: 28-18=10 & 32-11=21
Your fertile period lies in between these days. In order to prevent pregnancy you don’t have intercourse during this period. The more irregular your periods, the longer the period of abstinence will be.
So Barbara’s fertile period lies between 10 and 21 days after the first day of her period, as you can see on the drawing.
Some take one or two extra days at the beginning and end to decrease the risk of getting pregnant.