You're having trouble because you've started with an image of a duck in your mind that is abstract, not real. It's okay to change things from the way they are in reality to give them a little style or emphasize something you think is important, but you should start by looking at what a duck really looks like, and work from there:
Take a look at the overall shape, especially the silhouette. A duck has a very distinct silhouette!
Then, if you want to get more technical, take a look under the hood and see how its legs, wings, and neck are actually connected to the body under all those feathers:
Instead of trying to draw a cartoon duck you remember from your childhood, try to draw what you see in real life, but adding exaggeration to the parts you think are funny or interesting. That is true cartooning.
