Ovulation test

Hi all! Me and my husband have been trying to conceive for the past 1 1/2 years with no luck we thought we would give the ovulation sticks a Try as I have seen so many positive statuses and outcome due to using them, just wondering do I use one every day through out my fertile window? And if so how do I tell that it's the day of ovulation? (darker line compared to control line or both the same colour)

Thank you in advance :)