You don't try to draw something perfect right off the bat. You really just start scribbling and start having fun and thinking in terms of motion. And so let's imagine I'm drawing a character running. The first thing I'm always going to do is just get the idea across. Hit it really fast. And I do my best to ,sort of, make it all one drawing and it can be as messy as it needs to be.
I try to think of the body as one single, flowing shape, rather than as a bunch of different mechanical parts. Even it can be a relatively quieter motion, like walking. I still try to really, kind of, bring it to life.
A character can even be sitting and not doing much of anything. But still, just different cues of how they're sitting and how they're holding their body can sort of show what that person is feeling and give you a sense of life. The best way to do it is really keep, just practice doing these, kind of, loose, gestural drawings.
The important thing is not to get self conscious or to try let go of that idea. Try to let go of the idea that what you're doing needs to look right. Or, you know, try to forget about whatever mean art teacher who made you feel bad about your drawing in third grade. And try to just, you know, capture an idea of what a character is doing and then move on to the next thing.
One thing that helps me in this is that I've done a lot of figure drawing so I instinctively understand the proportions of the body. And when I'm drawing these quick lines I instinctively understand where all the bones are and all the muscles are. But at the same time, don't get caught up in trying to make things look right. Don't get caught up in trying to make that elbow look right. Just build this capacity to capture a quick sense of motion with a few quick lines and them moving on to the next thing.