This is a very hard concept. I have learned that if your position on the horse is wrong usually the position of you leg is wrong. And, you might be holding tension somewhere in your body, such as you lower back (I often hold tension there). You could also be grabbing with your knee, try letting your seat bones sink into the saddle, and think of bringing your sacrum to the front of the saddle, while allowing your legs to drop in a line that matches your spine to your ankle, while also keeping your legs relaxed, but not floppy, if that makes sense.
I hope this helped, it helped me with my leg position. It is very hard to understand without a visual.
Good Luck :)