OPKs First time using them!!

Sarah • Married, 36, 11 year old daughter, one angel baby, son 6 years old, and a baby boy 7 weeks old
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?