I can understand your confusion, and it is not an unusual scenario. Only textbooks have 28 day cycles. The important thing to remember is that it is the first half of the cycle that is erratic. After ovulation, you will have a period in about 14 days. Ideal conditions for conception occur before and during ovulation, and the best way to detect ovulation before it happens is by evaluating cervical mucus.
Fertile mucus is checked outside the vagina while wiping after every trip to the bathroom.
It stretches between the fingers. When it appears this way, it's the day to have intercourse. Attending classes in the natural family planning Billings method is the best strategy to use to maximize your ability to use it to achieve a pregnancy. Many cities have a local chapter, which would be happy to explain the method and get you a schedule for classes. I hope this helps.