To use the alla prima approach effectively, you should have a very clear mental picture of the subject, which means taking some time to examine it thoroughly. Squint at your subject and observe the light and dark areas. Notice the middle values that exist between them. Still squinting, make note of the general color in these areas. Look for the warm and cool color relationships; for example, if the subject is lit by north light, the light side of an object will be cooler than the dark side. Also pay attention to halation, or how the light values of the subject bleed into the darks and vice versa.
Starting at the focal point of the painting, I begin blocking in the main areas with paint, constantly comparing the subject with the painting and trying to match the colors as I see them. I apply paint with a large, soft brush in the beginning and gently soften most of the edges to establish a general feel for the subject. As I continue, I elaborate on the transitions between the light and dark values, pushing one color into another to obtain the proper edge. Refining the image, I gradually reduce the size of my brushes, painting smaller and smaller shapes and details.
For the best results with this approach, I find its important to paint deliberately and methodically. You have to take care to keep the light and dark values from mixing together on the canvas to prevent muddy colors. I use intermediate colors between the values to create the transition from light to dark.
The usual time limit working on an area is, of course, when the paint begins to dry. So I try to finish a specific area before this happens. Dont worry if you have to paint a little on dry paint to get it right. Remember, the goal isnt to complete a painting in a certain amount of time, but to produce a quality painting.