First period after miscarriage and when to start using OPK’s

I’m starting to feel like I’m about to start my first period after my miscarriage. I just have this strong feeling that it’s about to come. No real major cramping or anything but I have had some brown discharge so I’m thinking it’s coming. After I start and finish this period, when is the best time to start taking opk’s? I don’t want to wait too long and miss my window.. and with this being my first cycle after a miscarriage I’m not sure when ovulation will return. Before my miscarriage I usually ovulated within the week right after my period so should I just start testing a couple days after my period ends?