The first thing to know is the average length of your menstrual cycle. Once you know the length, (again for the purpose of calculating your ovulation calendar your cycle should be between 21 and 35 days long) you can
divide that length by 2 and round up where necessary, (e.g. if your cycle is 29 days, half that is 14.5 and 15 after rounding up).
The second step is to determine the start date of your next cycle (e.g. June 8th). Then subtract half the length of your cycle from that date, (e.g. May 24th). This is your next ovulation date.
Ideally you start trying to conceive 3 days before your next
ovulation date as sperm can survive up to 5 days (note the
free ovulation calendar will
show you all of this information automatically).