For the best results it is recommend to put the TENS on once you realise that you are having contractions rather than Braxton Hicks.
Be sure to start it on low and increase the intensity as required. There is a boost button on the side which, when pushed provides extra intensity.
It's so helpful to have the TENS on during the drive to your birthing location, as this keeps the happy hormones activated, unfortunately sometimes the hormones can decrease on the way to your birth location.
This is why Midwives recommend that you stay home as long as you can, to keep those happy hormones high, and the contractions rolling through.