Premix and load your white paint into the airbrush. The colors of the paint are going to depend on the colors of the feathers you want to create. If you are going for a bald eagle, then white and gray work very well. If you are trying to do a crow, black and blue work well, using blue for the sheen of the bird's highlights.
Use your small whisk broom (preferably one you don't mind getting paint on). Press the broom against the material that you are painting. If you are painting onto a canvas, press the broom against it so that the bristles of the broom spread apart slightly and you can see through them. If you can't see through them, no paint will get through, and if they are too far apart, too much paint will get through.
Spray the airbrush (you can usually find the lever or button at the top of the nozzle under your right index finger or thumb). Hold the airbrush with your dominant hand, and steady it with your other hand. Press it, and apply short, even strokes over the whisk broom's head. Hold the airbrush tip approximately 2 to 5 inches away, as this will result in a full but soft look to the feathers.
Remove the tip of the airbrush by unscrewing it. This allows you to expose the needle inside the tip. Once exposed, the paint will be able to be applied in a finer, thinner line than ever before---perfect for detailing feathers.
Take the broom off of the canvas, and set it aside. Insert and load the airbrush with gray paint in preparation for the detailing. Use the gray paint to give the feathers depth and outline---or any color, depending on the type of bird or feather.
Spray your gray paint onto areas of the feather that you feel should have depth. Use very short, quick strokes. Try to paint the center rib of the feathers in this manner, and then outline them as well. Don't do this for every feather, or it will look like scales, but be sporadic.
Combine your white and gray paint together to make a paint that will blend the softness of the white with the harsh lines of the gray from Steps 3 and 6. Spray the paint around the edges of your feather outline to enhance the depth of the image. Use this color to bring out the feather shape of the individual feathers or the feather clusters.