First paint in the sky, that will set the tone for the whole picture.  Put the high and low tones in each major area next, that is sometimes called the local color.  The highlights and deep shadows finish the painting.  Easy, 
right?  It's really easy when you paint in acrylics.  Always start in the top left hand corner, if your right handed. It's neater that way, you can crawl all over the picture with out making a mess.