Can somebody explain BBT to me.

Kenna
So I just started charting and I'm not sure how to determine this. I don't know if I'm doing this correctly. I wake up and take my temp as soon as I open my eyes. This is charting from CD 1. I thought it was supposed to stay about the same and then drop, but it keeps dropping. AF just left completely and it dropped a great deal to 91.8 which didn't seem right.. I took it again about 10 min later because the temp was weird to me and then it was 96. 4. I don't know if I should count the 96 because I was already woke but it seems more normal aligned with the other temps. Is this thermometer broken?? Am I doing this wrong?? Can somebody that understands bbt explain is this normal or explain what exactly it is I need to understand about these temps ??
CD1 97.7
CD2 97.5
CD3 97.2
CD4 97.5
CD5 96.9
CD6 96.4
CD7 Skipped 
CD8 91.8 / 96.4