How do you count DPO??

Briana❤👫👦👧🤰❤ • I`m 26 years old. Been with my husband for almost 7 years we`ve been married for 4 years. He has a 6 yr old son that is now my son too. We have a 2 year old daughter together and are now expecting our second girl. Due Fed 2019!
I thought you counted from the day after your fertile window. But someone said it's from the day after ovulation???