When to start using OPK's

Natasha
Hi y'all, just a quick question. I've been TTC for almost 2 years now, but I've, in the last couple of months or so, been using Ovulation tests. 
My question is: do I start using them 8 days after the first day of my period? I have a 32 day cycle (usually) and I read that it's 8 days after AF but it never said if that was from the first of my period or last day :/
I know it sounds slack but I wonder if I keep missing my window because I'm doing it wrong