Help please

Stephanie • I`m 24 years old and my husband is 31. We had our first born 12.01.16 and our second was born on 05.26.2018 #BlessedandThankful #MamaofBoth💙💕
I'm sorry for the dumb question that I am going to ask, but when counting DPO what day do you start counting? The day after your most fertile day? Or the day after your ovulation cycle or the first day? I'm just trying to figure everything out!