How do temp with bbt ?

I've never done that before I used concieve plus and opk got a positive opk cd 14 no luck this month 3 days away from AF and negative PT ... So I've just ordered pre seed for next cycle will arrive in five days. Should I go back on and order a baby thermometer ? It says standard centigrade or farenheit when I clicked to but one it came up those options and I don't know what a usual temp would be or a high temp