Questions about opks .

I never really understood the use of opks , until I look into it and did some research . I used to use the clearblue advanced digital ovulation tests , but it never seem to ever show anything but a circle . Recently , It broke , and I ordered the easy@home kit . I'm still pretty new to this .. When exactly in your cycle do you start using the strips ? Around what cycle day ? I've heard you should start around cycle day 11 , is that true ?