My horse has a bouncy trot, so I can feel your pain! If I'm not well balanced or tense I am all over the place when sitting.
While you're working on the sitting trot, try slowing your horse's gait down a bit. You'll generally have an easier time sitting, and once you get the hang of it more you can work on doing it at an increased speed.
As far as learning how to do it correctly, it seems everyone has a way that works for themselves, but not necessarily for everyone else. It's just finding that one way that sticks with you. For me, I try to think long. I think about keeping my leg long, weight in my heels, and my back straight and tall. I keep my hip angle open and try to relax my hips and relax into those "long" legs (keep in mind "long" is metaphorical... my legs are anything BUT long in reality!). I also work on not being tense. If I tense up, things will fall apart.
I'm sure others will reply with what works for them. Try them all out--hopefully something will work for you! Good luck!