I think what most people do is take them once a day starting near the middle of their cycle (I have a 28 day cycle and started testing CD11), and then testing twice a day when the OPK starts showing a faint line, until it shows a dark positive (for most tests, the test line has to be as dark as or darker than the control line). I'm just starting and might not be best the best to give advice (as I haven't even had a faint line yet!) but I didn't want to read and run!