OPKs First time using them!!

Sarah • Married, 31, 6 year old babygirl, one angel baby, Baby boy born April 11, 2017!!
So I've never used OPKs before but because this will be or 8th month TTC I decided to give them a try to see if the fertile window is when I'm told in this app. It's been super confusing though. Last night I took one and there's was pretty much nothing there (second from the bottom) so I took another one about 2 hours later & it was much darker. I really don't want to miss my surge... What do you guys think?