I know this amazing technique that rotates the hips/pelvis and overall helping ur horse become rounder. It's like a pressure point kinda thing. There a video that has it video: 8 different horse stretches by Evention TV. I also releases pressure on the muscles on his back
Honestly, one of the best ways for someone untrained to massage a horse's back is to give them a good brushing with a rubber curry comb daily. It gets the blood circulating, as well as getting rid of loose hair and dead skin build up.
Carrot stretches are good in moderation. Always keep in mind though that a horse will over stretch to the point of doing damage if they think they'll get the treat. You have to be responsible for the amount of stretch. Start with small stretches and build up slowly. If a horse gets "stuck", don't push them through it, they likely need a more qualified person to free them up.
There's a few legs stretches that can help with the back. Picking up the front leg and bringing it forward until it's a few inches off the ground in front of the horse. Wait for the horse to relax and straighten the leg before putting it down. You've probably seen people do this much higher but unless you knew what you were doing, I wouldn't chance going higher with the leg in case of causing damage.
Back legs can also be lifted and moved back. The arm closest to the horse should go around the lower limb under the hock, to support the fetlock with your hand from the inner leg. Then slowly walk out with the leg. Again, keep it low and do not go further than the point of resistance, which is not very far in a horse who isn't used to stretches.
Never stretch a horse with cold muscles. Do it after working them so the muscles are warmed up. If you want to do a deeper massage that the curry comb, warm the muscles up there first. Cold muscles are not elastic and manipulating them will cause damage.
If you are unsure, get people to show you. Best ask than agitating a potential injury or even causing one.