OPK help

Danielle • Momma of a boy, engaged to my best friend, trying for baby #2 since June 2017! ❤️❤️

We've been TTC baby #2 for 8 months now with no luck. My cycles have been irregular, I've only had 1 period since august (period lasted 20 days😳). I ordered some ovulation tests, but I've never used them before so can somebody explain them to me? Do I use fmu like with a pregnancy test? When should I start taking them? I'm so confused.