How do I know when my fertile window has started?

This might be a silly question but how do I know when I’ve entered my fertile window? I know you are supposed to BD every other day until a day or two after ovulation. I’m just unsure of when to begin BD.This is my second month <a href="https://play.google.com/store/apps/details?id=com.glow.android">tracking ovulation</a>.