Opk questions

Can anyone explain how to use opks?

Do i start testinh the day my period ends?

Do i test multiple times a day or once a day, how often should i test?

How do i know when the line is darkest when they look the same a lot of times???

Do you always test right before you baby dance to be sure or is there a good time frame??

Are u usually same every month or is it always different?