It depends on how long your cycles usually are. If you have very long cycles, you can wait until later, if you have short cycles, start earlier. I usually start using OPKs around CD10-12.
I use Wondfo brand. I buy them on Amazon and get a LOT for a very reasonable price. 100 tests for around $100.
I test once a day in the afternoon. For me, personally, I know that I get the best chance of catching my surge in the afternoon around 4pm (I discovered this by testing twice a day for a cycle). If you have a long surge then it probably doesn't matter what time you test, if you have a very short surge sometimes women will test twice a day.
You want to have a several hour "hold" - meaning it's been a few hours since you went to the bathroom and consume limited fluids during that time.