It depends on how long your cycle is to when you start to use them but as yours are all over the place i would start using them from CD14. As you don't want to fall pg just yet once you get a dark second line stay away from DTD. As once that line shows up you are due to O in the next 24-48hours.
Depends on were you are buying your tests they should have a good instruction book with it and a list of dates when to start using them.
They say use them between 10am and 10pm. I always did mine around 2pm and made sure i had nothing to drink for a least 2hours before.