First time OPK! Help!

Mi
Hi girls,
​I plan to start with opk testing this cycle but I don't know when to start. I have PCOS and have regular cycles because I use progesterone from 18 till 28 CD. My doctor said that I start this month from 15 till 25 CD because I start to bleed on 15 CD every cycle.I think that I don't ovulate but I have my periods. 
​I also started bbt so we'll see. 
​I'm CD 4 today.
​Sorry for this long post but I really need help!
​Thank you!! ☺️