Tens unit works great for the muscle spasms I have. However it doesn't seem to do a thing for the broken bones and torn hip flexor. I think it depends on the type of pain and the location. As far as instructions, you can usually find a manual online if you google the make and model.

My instructions were to increase intensity until you found one that is strong but not painful and play around with the different variations until you find one you like.

Good luck. Hope it works for you.