How do you count the fertility period of a woman after menstruation?

You count the first day of your menstruation as Day 1. On average your ovulation takes place on/around Day 14 so if you're unsure, you could start testing for your ovulation from Day 11. From the moment you have a positive ovulation test (which is meant to show the increase of hormones leading up to your avulation!), you can expect your ovulation to happen within 24-36 hours.

From the moment the egg is released it can only survive for about 12 hours. A man's semen can survive for 2-3 days which is why it's advised to start having regular intercourse in the days leading up to your ovulation. For the highest quality of semen at least 1 (male) orgasm per day during the week leading up to your ovulation is advised.