I caught a ball just inside the boundary. As I caught it I took one step back over the line. I caught it cleanly. Was it a six or out please?
From your question I gather that you were still moving as you caught the ball. If this is correct, the batter is not out and it is a six.
As per Law 32, a catch is considered to be completed when “when a fielder obtains complete control both over the ball and over his own movement.”