Set up your workstation with all of your tools and paints, and place your predrawn pinup picture in the middle of your workspace.
Place a sheet of frisket film on top of your pinup picture. Using your razor knife to trace the outline of your pinup through the frisket film, create a stencil of your pinup. Carefully peel away the unneeded piece of frisket film and put it off to the side of your workspace. Leave the desired piece of frisket on your picture so that it is ready when you begin airbrushing.
Fill your airbrush with your desired color of acrylic paint. Do a couple of test sprays on your piece of scrap paper to make sure that your paint is the right color and is traveling smoothly through the airbrush.
Begin airbrushing the area of your portrait that is uncovered, using your frisket film as a guide to where you need to airbrush. Wait a couple of minutes for the paint to dry.
Remove your piece of frisket film from your newly painted picture and touch up any mistakes with your fine paintbrush and acrylic paint.