Center of Interest
Start with your center of interest and really nail it. Work out from there blocking in the largest shapes first. This I found to be the best approach especially out in the field, "alla prima".
The sky went from a few clouds to over casted in a short period of time.
Here I did some pencil sketches to get an idea of compositions.
Then I went to water color starting with the center of interest knowing what my composition was going to be. Making color notes and photos for reference, I will go from here into the studio to work in oil paints.