Test the your airbrush machine. Fill the reservoir of your airbrush machine with a small amount of any paint. Spray a piece of paper towel. If the paint is not passing through the airbrush machine fluently, it's too thick. Dilute the paint with a little water, and repeat the test until the paint dispenses evenly. Also, test the pressure abilities by testing the trigger of the airbrush machine.
Clean the reservoir of the airbrush device with water. Make sure it is clean by spraying a piece of paper towel, like that of the test. If color appears on the paper towel, it is not clean. Continue to flush the airbrush machine with water until the paper towel comes clean when it is sprayed. Repeat this step every time you change colors. This is imperative because mixing the colors due to an unclean machine will ruin your design.
Pour your base color into the reservoir. Apply it to your canvas, and cover it completely. Paint two layers to ensure a solid, strong, even coat. Flames tend to look better when the base color is very dark, as it provides a nice contrast to the bright colors of the flames. Black or something close to it is the suggested base color. The canvas used is left to the artist's discretion. Allow specified time to dry, depending on the material used.
Refill the reservoir with the blue paint. Place the flame stencil on the surface of your canvas in the direction that you want the flames to shoot. Airbrush the blue part of the flame onto the canvas. Only use the blue paint at the bottom of the flame, as with most fires, only the bottom or the center is blue. Allow time to dry.
Clean out the airbrush machine and test its cleanliness again. Pour red paint into the reservoir. Apply the red paint to the design using the stencil. Use the stencil liberally to enhance the look of your flame design. Unlike the blue paint application, paint the red all the way to the top of the flame design, as you will go back and enhance it with the lighter colors. Allow time to dry.
Paint highlights on the red parts of the the flame design with a bright orange paint. Make sure you are following through with the proper cleaning procedures of the airbrush machine between color changes. Feel free to use the stencil more with this color if necessary. Do not use so much of this color that the ones underneath are no longer visible. Allow ample time to dry.
Paint around the sides and the top of your airbrush design with the bright yellow paint. Like that of the orange paint, do no use too much color. The red, orange and yellow paints should have a good balance in the flame design. Allow to dry.
Place the smoke stencil along the top of the flame design, barely overlapping the top of the fire. If a stencil is not used, lightly draw curved, smoke-like lines using the gray paint. Do so lightly, as the smoke has to look authentic. Paint the smoke in gray as desired. Allow to dry.
Paint faint highlights along the gray lines of the smoke with the white paint. This should be the color that you use the least. Allow time to dry.
Clean out the paint reservoir of the airbrush machine one last time. Close all paints tightly and store in a cool, dry place.