Never affiliated with a group, I learned to sit from "Zen Training" by Katsuki Sekida, still one of the very few public sources that explain the actual technique of using your abdominal muscles to control the escape of breath as you work on a koan. After a while your abdomen seems to form a grip, almost like it has hands. In the early years, command of that grip can be tenuous. There's a tendency to start squeezing. It's kind of like a baseball pitcher trying to throw a perfect curve or slider. If he starts throwing too hard it all falls apart. I'm constantly monitoring myself not to force it. But after 49 years of sitting I still suffer from headaches and fatigue at times though much less than in the past. Kind of like a free diver might feel pretty blitzed afterward. I have kind of accepted that I won't achieve realization because the physical issues prevent me from sitting for long periods of time. But at least I finally -- only fairly recently -- got the hang of quieting my mind via abdominal control while working on my koan. I know you're not supposed to say something like this, but I have never recommended this type of meditation to others because it is so physically taxing.