OPK's and Cycle Days

Kelsey
Cycle Day 38, pregancy test today was negative, and no signs of AF coming like I usually have. 
Curious about OPK's since it seems my cycle and Glow don't really add up. It's so frustrating because as my cycle gets longer, my fertile window shifts and then those days have already passed and you can't make up BD after the fact, so... I'm going to try OPK's this cycle but I don't know how to use them? Do you start testing immediately after AF? How many do you take a day/week? 
I regularly chart my BBT and CM but my cycles have been longer around 32 days avg but my last one was 28. We're on Month 6 of TTC, but haven't really been all about it. Moving, school, graduation, etc. got in the way. Tips, advice, and prayer appreciated!