It’s very easy, if you start by simply having them record their dates of menstruation, as soon as it’s relevant. If they have unusual results, then observing temperature can often be informative.
The menstrual cycle is a general reflection of a woman’s health from puberty to menopause, and it’s very unfortunate that so few women, and so few doctors, pay any attention to what it reveals.
I had 21 day cycles as a teenager! That was not fun. Now, looking back, I’d be curious as to how long my luteal phase was with a cycle so short. I have found temperature to be kind-of a pain (I wake early for work), but I can pinpoint ovulation based on the other methods.