Download the free image manipulation program called Gimp at gimp.org. Install it after the download.
Open Gimp and create your Airbrush design. Be sure to use the spray-paint type of brush to give the design the airbrush look.
Save the design as a JPEG, name it and export it to a familiar file location.
Go to Printfection.com, and create an account. Go through all the confirmation such as confirming your email and other important information.
Log in to your account and select the "Design a Product" option. Choose a type of clothing you would like the airbrush design you've created to appear on.
Choose "Upload Image" and select the design you created in Gimp earlier.
Choose whether you want the airbrush design on the front or back of the clothing and confirm.