How do you bbt test? I'm thinking about trying it? How is it supposed to look when you ovulate? I'm kinda confused how it works

Ky 8-15-17👼🏼 & 10-28-17👼🏼 Pregnant with my🌈💕
Anyone who does bbt testing, how do I do it? I've been trying opks but am on CD28 and still haven't gotten a positive and am thinking maybe I'm missing it. I'm kinda wanting to try bbt testing but I don't know how to do it and how it all works. Anyone help? I ordered a bbt thermometer to use also, by the way.