Choose a photograph to work from. Maybe you are reading this because you already found a photo that inspired you. Look through your own snapshots or search the Internet. It is fine to paint from a picture in a magazine for your own enjoyment, but if you want to sell or publish your painting, you must ensure that the image is licensed for others to work from. You can also do an on-site color sketch, if you live near a desert; this method provides subtleties of colors that a photograph will not show.
Look at the photo and think about what makes it so inspiring to you. Make a black and white copy of the picture so you can see how the values (darks and lights) work together.
Cover the edges of the picture with pieces of paper to see if it would look better cropped. If so, crop or tape off the edges as you choose.
Outline the major shapes to get a feeling for the composition. Use a dark pen or marker on a spare copy of the photo. You might outline dark and light areas that are part of the same object.
Do several quick sketches to get a feel for the subject matter and composition. Cut your sketching paper so that it has the same height to width ratio as the canvas that you will paint on.
Look at the various color relationships. Always compare areas of color in the photograph with the same areas in your painting. For example, the shadow side of a rock will be darker than the sunlit side, but really look and see how much darker it is. See if the shadow is bluer or redder than the sunlit side.
The Southwestern landscape is characterized by its particular type of sunlight. This is most easily expressed in the relationships between sunlit surfaces and shadows. Shadows can be surprisingly rich in color; look at them carefully. Do some color sketches of lights and shadows so you will get a feel for how to mix your colors.
Now that you have become familiar with your subject matter, take out your canvas and start painting. Don't get bogged down in detail. Instead, work quickly and cover the whole canvas filling in more and more details as you go.