First time using opks how do I use them?

New to the group thank you ladies❤️ This is going to be my 12th month of ttc and I never used opks before I'm going to buy some at the dollar tree this week my period just ended February 24-28 and I'm hoping March it will happen cause if not I would have to go to a fertility specialist which I don't want too because I know it can be costly😭 thank you for advice I'm irregular so I have no idea when I ovulate