TAZMZN gave you some good advice, you should get some help from a professional on how to use it. As far as the burn spots, this could have been the skin reacting to the sticky pads that you place on your skin, some people may be very sensitive when it comes to their skin. There are some things that you can put on your skin to prevent this from happening. Another problem which could cause burns would be a defective pad. I never had any problems with my skin or burns, and I sometimes maxed out the intensity knobs, but every one is different. I used mine 2, sometimes 3 times a day for 30 minutes at a time. It sends electrical messages to your brain that hopefully fools it and that results in some pain relief. It worked wonders for me, and I absolutely love it. I am not sure if you can over use it but if you feel that you need to use it more than 2 or 3 times a day, you need a lot more help than the tens can provide.
Mine had two channels, two pads per channel. You do not need to use both channels with all four pads, but I usually used all four. I would crisscross the pads to surround the area that I was trying to relieve the pain for, which was my lower back. This is how my PT told me how to use it. But you have to be careful as that little nine-volt battery packs a punch. Turn those intensity knobs very slowly at first until you find a comfortable pulse or current. Mine has three modes, a constant pulse, a kind of a wavy pulse, and then it has the burst mode. I didnít like the burst mode, I usually used the wavy mode. Everybody is different and you have to find what will work best for you. My PT said you could use it on most places of your body, arms, legs, back, but never put the pads on or around your head or heart. If you acquired this machine second hand and do not have new pads you should definitely go out and buy some new pads, I wouldn't recommend using someone else's pads. They are easy to come buy, just call your local medical supply store.
Again, this is my experience, but you really should have a professional teach you how to use the machine properly to get the best results as TAZMZN pointed out, you may cause more discomfort than you already have, and that would be terrible.
Good Luck and God Bless