no clue how 2 track ovulation!

Lea

so i been using this app sense spring last year an my cycle days have a huge variety. last 6 months are. aug. 35 day cycle sept 39. oct 42. nov 42. dec 26 jan 35. how can intrack best so i can narrow down my fertile window? the app does help but considering my cycle days are always dofferent i dont fully trust the app.. thank u ladies if u can help me figure out a easier way 2 track it. also fyi. i am NOT on any birth control or medications