Track the first day of your period each month
Count the number of days from the first day of one period to the first day of the next period
Repeat this for at least 3 months
Add the cycle lengths together
Divide the total by the number of cycles
Use the average as your monthly period length
If your cycle is irregular, use the shortest and longest cycle lengths to estimate the range
Record the dates in a calendar or period-tracking app
Update the calculation each month as new data becomes available