Here's a demonstration of gedan uke.
So, as you're doing gedan uke again, just is in jodan, you want to start from your basic posture and again, rotate your arm downward, from the shoulder, as you're starting out. One simple movement,from the shoulder. Make sure, you're rotating completely, from the shoulder. Okay?
And again, use your body motion, as you get more and more advanced. And then eventually, add your Sabaki Gata or Leg Movement. This is gedan uke.