how do OPKs work? help!

Jessica
We've been trying for 8 months now... I see everyone using opk tests... how do these work? Do test in the morning & at night each day? 3 times a day? I'm so confused! Also, what does everyone use? Is it this one ( https://www.amazon.com/gp/aw/d/B01N8U3LQU/ref=mp_s_a_1_1?ie=UTF8&qid=1497012353&sr=8-1-spons&pi=AC_SX236_SY340_QL65&keywords=opk+kit&psc=1 ) from Amazon? Please help, fill me in on the details!
Thanks so much!

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