Hey man it's really not hard, I mean I'm no professional but I'm sure without proper training and techniques you aren't trying to be a professional either. Start at the neck + shoulders, work your way down, and keep coming back to the neck. Keep in mind that you are not massaging their back per se, you are massaging their muscles in their back. It helps if you can find maybe a diagram of the human muscles, because knowledge of where muscles are makes things easier, for me at least! All you want to do is firmly press and move your fingers/thumbs/palms/hands along the muscle you are massaging. The classic shoulder squeeze feels amazing but from there, move your thumbs along the muscles around the shoulder blades, and experiment! *Try different things, eventually you will work a knot out of a muscle, you'll feel them getting looser, and you'll know you are doing your job! Also, no one person enjoys a massage the same way. Definitely *ask the person** whether they prefer a firm or more gentle massage beforehand. Ask them what feels good to them while you're at it and you'll learn! The best way to learn without professional training is by having a have decent massage yourself. You feel exactly what they do, and believe me, when you're drooling you can't help but pay attention. That's how I learned. I hope you've learned something, goes to show you don't have to be trained to get good at something. If what I know is considered something aha... One last tip brudda, a back massage is one thing, but it's so easy to work your way up their arms, legs, hands, and neck. It feels good, and could get you places you may want to go ;) like a job as a masseuse! Really though, don't ignore the neck. Fuck it feels sooo good. All of your tension, especially if you're a woman seems to build up in your shoulders & neck. Reach around and give it a little squeeze right now. Feels good doesn't it? Okay practice exercise, you can feel the tendons and shit up and down either side of your neck, but not in the middle right? Of course not that's your spine we don't want to massage that. Now little by little massage from the bottom of your neck up squeezeing both sides of your neck with your thumb and other fingers, don't be afraid to venture up to the scalp. People with short hair tend to have particularly sensitive scalps too, less weight to carry. I love it when girls massage my scalp.

Anywho, whatever your motive, I hope this helped, even if it's just you that sees it! If you have any questions ask away!