How do you use the basal baody temperature?

I think you mean basal metabolism or basal metabolic rate. The body temperature is supposed to stay pretty much constant -- unless you're sick.

Basal metabolic rate - Wikipedia, the free encyclopedia

Here is just one of many sites you can read through. A couple of things to keep in mind however... it can take a few months to determine a pattern of when you most likely are ovulating... you need to be consistent with temping and charting... it doesn't mean you definitely are ovulating, but that your body is preparing to ovulate... start trying frequently several days before you suspect ovulation may occur.

Charting Your Basal Body Temperature Information and FAQ - Fertility Plus