Every painting begins by blocking in the composition. There may be as many variations of how to do this as there are painters, but we still share the basics.
In class, we talked about drawing in the lines of the composition, either with paint, or water-soluble fabric pencils or fabric markers (whose lines will dissolve when we come along with water and acrylic paint). Then you begin blocking in the big shapes of your painting. It's all very rough at this stage - you don't want to even think about the details yet.
Painter Jerry Fresia, originally from San Francisco but now living in Lake Como, Italy, has posted a great example of how to block in a painting in "A Painting Class on Lake Como, Italy." While he is painting in oils rather than acrylics, and begins drawing his composition with charcoal initially (vine charcoal is often used by oil painters for this), his process is basically the same one I demonstrate in class.
Notice how he also chooses to scumble in his colors initially (not as necessary for acrylic painters as oil painters) and how he works from darks to lights. He breaks down his painting process into different stages of development that are helpful to see. And while you're watching, enjoy the views of Lake Como and the beautiful castle he lives in!