When you first observe a new subject keep in mind that you can make any changes you want to improve the painting. You are the one telling the story.
A good first step is to decide which shapes need to be separated from each other to easily understand where they are in space. Try to keep the number of shapes reasonable, say no more than ten or twelve. (the assumption here is that you intend to create an effective illusion of depth).
Once you have identified the major shapes in a photo or a scene, I recommend outlining them with light pencil lines. Keep it very simple, though. The role of the pencil in a watercolor is to make it easier to apply the paint with confidence, freeing your brushwork as much as possible. Too much drawing can have to opposite effect, making it seem like you must stay inside the lines, thereby constraining the brushwork instead of liberating it. At this stage of the painting it is not necessary for the hypothetical viewer to know the identity of the shapes. This is when we only need to know where they are relative to each other.
Now is the best time to consider relocating or removing the shapes. This is a process that involves your instincts as well as your intellect. In the image below, the horizon is above the middle of the page, giving dominance to the foreground. What would be the impact of lowering it?
Let's take a look. While we're at it, I'm curious how the feeling changes when that little out-building on the left is removed.
I'd be inclined to stretch the sky upward even more, to increase the feeling of loneliness in the scene. What about eliminating the smaller barn altogether?
If you don't happen to have a handy pocket composition adjuster, you do have a simple outline drawing of the major shapes, and, hopefully an eraser. You can see the benefit of making these changes before you start putting paint on the paper.
Here's a photo with several shapes:
Kinda crowded. What might you do about that? Also, the bottom left quadrant feels empty. Could you add something there that would balance the composition without adding to the shape jam?
Take a moment to look for anything that creates a feeling of ambiguity regarding the illusion of depth. This would be the right time to move things around to make it easier to tell what's in front of what. Hint: Close one eye and cover the buses and the fronts of the cabins below the roofs.
Here are a couple more that may need some adjustments. Make your simple drawing of the major shapes and move them around as needed, adding and subtracting until you are ready to put some paint down.