Mawatte Gedan barai has two important components, which should be connected to each other.
First there is the turning of the body, with the body. Don't think of stepping with your foot, but about turning your body. Also, make sure it is one movement.
Execute Gedan barai as you turn your body. Make sure to do Gedan barai in a smooth -that means one- movement. Keypoints of Gedan barai are using the elbow (pull your elbow) and the twist of the wrist.
Since Gedan barai and your body move approximately at the same time, you can use the build up energy of your body movement to make Gedan barai more effective.
I will explain and show the above mentioned more thoroughly in the video.