I have been using an OPK for almost 3 months and never got a positive until last Friday. Exactly the way Mumita did it, I did. Got cramps friday morning, went home friday afternoon and wala the line was as dark as the control line. BD that night, the night after and on Sunday and now I am in the 2ww. My first positive OPK, which my silly husband mistake for a pregnancy test and was so happy until I had to burst his bubble and tell him its a positive OPK...
When I used the OPK's with lines I had a 2nd faint line every single day that I tested! So I'm not sure what to tell you! I have heard though that for some people the line will get steadily darker as they approach ovulation, and I think that's only a few days beforehand.
Just pee on one everyday until the test line is as dark as the control line. And I guess bd everyday that the lines are showing on the test.
I can't tell you because I used one OPK stick once, I had a cramping on my left Ovary and I peed on the stick and the line was dark red. So I bd'd that night and the morning after and I'm almost 8 weeks pregnant.