How can I be safe to protect against nerve damage when I use my TENs unit?

My doctor gave me a TENs unit to take a away my pain two months ago, and recently he advised due to the progressive and degenerative nature of the disease I have (mitochondrial disease, it affects my muscles) to use the device as a way to build muscle and excercise. For this past week since I've used it every day on my legs (top and bottom), my hips, and my back for thirty minutes. I've noticed instantaneous improvement. I can walk for longer periods, I've been getting more toned, and my pain is almost nonexistant! However, I am aware electrical stimulation can be dangerous. How can I protect against nerve damage when using this device? I don't want any more problems haha.