Confidently riding one-footed takes a lot of practice, and I don't claim to be an expert at it. I do know that any rotation in the upper body will eventually transfer to the board (pivot). This often results in a heelside washout, so keep the shoulders steady relative to the board.
Also, I can't edge aggressively with my free foot. I have to be gentle with the free foot. Instead of trying to apply pressure to my edges per se, I pressure more toward the center of the board, but on the heel or toe side of center to shape turns, like I'm using the back of the ball of my foot and the front of my heel instead of the ends of my boot. That may be a clumsy explanation, but hopefully it makes some sense. It's just a different mental approach to steering the board, but it has helped me.