If possible, start with the lighter colors and work up to the darker colors. You will get better coverage with fewer coats that way.
Are you going to mask the different colors or spray them freehand? Do you want a sharp division between colors or a soft blend?
Assuming that you are going for a "Soft Line" camo job and spraying freehand (no masking), draw the pattern lightly on the model using a pencil. Keep the pencil lines about 1/8" inside what you want to paint to leave yourself a little room to paint over them.
When doing the divisions between colors, thin your paint and spray at as low a pressure as you can to still get good coverage. Keep your airbrush pointed in toward the color you are painting so that overspray doesn't get on the other colors. Keep an eye on what is behind what you are spraying, and cover it with a towel or something to keep overspray off. Get very close to the surface, use a very light trigger finger, and go slowly with the lines. Make your division lines as smooth and even as you can. Outline an area, then go back and fill it in.
Once the second color is done, you'll most likely need to go back to the first color and "Tighten Up" the divisions a little. Again, keep the airbrush pointed so that overspray doesn't get onto the other color, start about 1/4" away from your division and paint your way in toward it. This will cover up some of the overspray and make the demarcation tighter. You may have to repeat the second color again in places, but the demarcations will close up quite well.