Figuring out ovulation

Kendra
We've been TTC for about 4 months now. 5 months ago I removed my Mirena. Every month I use the OPK and have sex the day before and the day after a positive result. No baby yet...and I'm kinda sad. My cycle has been slightly irregular...22 days, 26 days, 25 days. How do I know when to do the do? Any advice? TIA