Just as with heads, you should make a rough sketch of the body before drawing. The image above is a rough sketch of an old man with a walking stick. As you can see I use three different techniques. I generally use them in specific styles and situations. For example, I'll usually use the second technique for more realistic drawings, like comic book heros. For looser styles, like the funny pages, I'll usually use the others. But, like everything in cartooning, there are no rules about it written in stone.
Is this really necessary? I asked that same question when I first started. And, after not getting very far, I found out that the answer is yes. I think that failure to sketch first is one of the leading factors in some one not sticking with cartooning. Well, I know some cartoonists don't do this. But they are probably drawing characters that they have drawn for years. You don't have to take my word for. Omit this step if you want to. But remember when I said cartooning takes work? This is a part of that work. It is like building the foundation for a house. You don't really have to build the foundation. But, how good of a house are you going to build. It's up to you.
Moving on, we come to the first figure above. A simple stick figure. Before you do anything you need a plan. That is what the stick figure is. A plan. When you draw the stick figure you can visualise the final result. And you can revise the stick figure. Instead of finding that you need to change the cartoon after you have drawn it. If you are drawing in a very loose style the stick figure may suit your needs. If not you may need a more detailed plan.
The second figure above adds ovals to the stick figure. Think of the stick figure as bones, and the ovals as flesh. Now just put a skin on it. For more realistic cartoons you can use more ovals than I have here. You can have an oval for biceps, pects, delts, whatever(if you want to draw comic book heros, find out what these words mean.Hint: they are muscles).
The last figure above uses triangles. I used triangles. You can use rectangles. Or a combination of both.
There are many more techniques that cartoonists use. These are just the three that I use. Which one I use depends on the style of the cartoon(among other things). There is no concrete paradigm for when to use what. You just have to experiment to see what technique works for you in a given situation.