What Is The Usage Procedure For TENS Unit?

I suggested to a friend w/ low back pain that a TENS unit might possibly help her as it did me for 20 yrs. In my case, a nurse came out & did all the settings & told me how to use it, telling me to never leave it on for 20 min. or my brain might catch on to the trick. I did that religiously (have a MS in neuroanatomy, so the fact that the sensory cortical representation of low lumbar spine could only perceive one sensory modality, either that triggered by C-fiber endings & interpreted as pain , or pleasant tingling sensation at one time, seemed credible to me & it worked, permitting me to travel on business a lot & sit & stand for hours at professional meetings. could not have done that otherwise). My friend got hers in the mail, can t figure it out, wants to ship it back, said there were no instructions about only leaving it on periodically. So I googled how does TENS work? , hoping to find some useful info. I could send to my friend. Can you provide a useful reprint on the use of TENS that I can send her?