It's easier to get the feeling riding without stirrups - if you brace against the stirrup, you lose the following seat. Best method is to be put on a lunge line, without stirrups, with a strap attached to the dees on the front saddle, or grasping the pommel. Get someone to check your position, and make sure your seat bones are directly underneath you and your back is flat and relaxed.
Pulling your self deeply into the saddle with the pommel or the strap may help you feel the motion. Then you'll need to work on just softly following, not driving. If you can get it on the lunge line without stirrups, it will then take some work and practice until you can transfer the skill to off the lunge and with stirrups.
If it's the motion itself that eludes you, Centered Riding by Sally Swift has some great illustrations and explanations. You'll feel like a perfect idiot doing "Monkey position/reverse monkey position" from the book but it does work. I saw a Centered Riding clinicianr make everyone get off their horses, and run through the exercises and the motion on the ground. Pretty funny to watch, but everyone was riding better at the end of the day. Good luck.