First, I listen to the soundtrack of the dialogue for the scene a few times and just start to visualize the scene as I am listening to the actors voices. I will imagine several different possibilities of how the scene could play out and determine which of those possibilities is the strongest for what the scene is trying to convey. Then, as I am listening, I will start to number the frames on my x sheet as I am imagining them with the dialogue. As I do this, certain key extreme poses will begin to stand out to me, and I will then begin to map out those extremes on paper. Those are not completed drawings, mind you. I am just putting down a few lines, as few as necessary, to just establish what the pose is. From that point, I will start to imagine what movements need to take place between that key pose and the next one. Then, just like before, I will map out those poses. This is essentially a graphic shorthand, symbols of head shapes, hand shapes, and joints that enables me to get my ideas out in front of me in a visual format so that I can see the patterns of action that are happening in the acting of the characters.
From there, I go back to those drawings and make that shorthand stronger and more clear into actual poses. I might end up with 9 or 10 key extremes this way. Then I take those, number them, and expose them on my x sheet, matching the pattern to the words where that dialogue happens in the scene. From there, I will shoot those extremes and look at them to get an impression of what the animation is looking like. If it looks good, I will move on to the next stage which is to refine the extremes and start to add in the breakdown frames in between them. Once I do that, I then look at the scene to ensure the necessary patterns of movement are all there and then shoot the scene again as a final pencil test for review. That is basically my process in a nutshell!
While doing all of this, it is also important to keep in mind that action is what matters most in animation. When you animate, you are becoming an actor, and you are using your pencil to convey your acting to the audience. Creating action is the goal, drawing is simply the means by which we create the action. It is easy to get caught up in thinking about your drawing, but you want to focus on the acting and how the mechanics of animation apply to that acting. When animating, your brain must be in an entirely different mode than it is when you are drawing. If you approach animation with your brain in drawing mode, right off the bat you have approached the problem solving process of animating with the wrong set of tools in your hands.