OPK Questions??..

Ni

So I bought a OPK kit and I really don't know when I should start testing. Glow has my first day of my fertile window for next Tuesday. Should I test beginning that day? Or before then? Any tips? I've never used these before this will be my first. I usually go off my CM but I had a D&C; and my first cycle so I don't know if my body will do what it used to do before I had the surgery sooooo I figured a OPK will help.