The most effective way of learning to recognise your individual pattern of fertility and infertility according to the mucus signals is to record your observations each day in a chart. The aim is to identify on any day whether intercourse could or could not result in pregnancy.
Watch and listen to this introduction to the Billings Ovulation Method®.
This animation explains the changes in cervical mucus that you may experience throughout your menstrual cycle. Once you're familiar with these changes you'll be able to identify the fertile and infertile times in your cycles.