Much has been written about where an artist should start their painting. There is no right or wrong way but each way has its particular advantages and disadvantages. I prefer to look at the question more from the point of view of where you need the most help in your painting design and composition.
The Center of Interest: for those of you who have difficulty creating a strong center of interest in your painting this method my work for you. Start with the center of interest first. Make sure this area is delineated at a higher level than the rest of the painting (best detail), has the strongest colors, has the greatest contrast, has the sharpest edges and has that special impact that readily draws the viewer’s eye to it over the rest of the painting. Your painting is a concert and not all parts can share center stage. By starting with the center of interest it is easy to tone things down a bit as you precede away from this area painting loser further away, using not quite as bright of color, using less contrast range and using less detail. This method insures a good center of interest is created but may not show a good sense of depth in the scene.
Background to Foreground: Because things are generally lighter and cooler in value and more vague in detail in the background some artist, especially watercolorists, find this method of starting works better. Start with light and cooler elements in the background and continue coming forward in the scene as you paint the scene. Colors should get warmer and brighter as they come closer. Contrast should be stronger (lighter lights and darker darks) as they come closer. Detail should be more apparent in elements as they near you. This method insures a good sense of depth in your painting but may not insure a strong center of interest.
Top to Bottom: Some artist prefer to start from the top and work their way down the painting. This method seems to work for those who have prepared a value sketch to guide them through the process. They have worked out their compositional elements in their sketch and know where their center of interest is going to be. This method seems to work for more methodical artists with a lot of painting experience under their belt allowing them to adjust as necessary on the go. This method may not allow a watercolor painter with transparent colors the necessary adjustments required for the impact one element has on another.
The Most Difficult Area: Other artists prefer to start with the most difficult area of the painting first to set the tone of the piece and use that initial enthusiasm a painter has at the beginning to help tackle the toughest part first. Paintings with people in them can particularly benefit from this starting method. If you don’t capture the mood and physical character of the figures from the start the painting is lost. The drawback to this starting method with watercolors is that you then have to proceed to “tuck” other elements in around your figures.
Blocking in the Entire Area: Another method is to start by blocking in the entire painting. This brings all the elements of the painting up at the same time. Again, a strong value sketch helps the artist using this method to guide the composition along. The main advantage of this method is that it allows the artist to judge how each component affects the other pieces. Again your painting is a concert and every part affects the other parts. By blocking in the whole painting you can begin to judge these affects and adjust for the balance you desire as you proceed to create a stronger design. The drawback is that you have “tuck” additional elements into the whole to achieve balance.