Ideally you would only use Ovulation detectors on the days leading up to Ovulation, which typically is between 14-16 days before your period is due. However if you have irregular periods, it will be hard for you to know when to start testing and daily ovulation testing is very expensive!
I would reccomend you start noting your "fertility" symptoms - Such as taking not of your CM (Cervical Mucous) and eventually checking your cervical position (Alhough this takes some getting used to!) - These are the best indicators of when you are most fertile.
With your Cervix you are looking for it to be High, soft and open.
You want your CM to be one of the following:-
Creamy, Milky, Lotion Like - FERTILE
Cervical fluid now becomes much more abundant and will feel wet from 2 - 4 days. It can appear either thick and creamy or like hand lotion and will form peaks in your fingertips when pulled apart or can be thinner and look milky. Color of the fluid is white or yellow and the overall vaginal sensation is wet.
Eggwhite Cervical Fluid - VERY FERTILE
Cervical fluid now reaches its most fertile stage for 1 - 5 days. The fluid will look and have the consistency of eggwhites. It is slippery to the touch and if pulled between the fingertips
Hope this helps!