Design the outline of a fish. The design can be as simple or as complex as you would like, but keep in mind that you will be cutting it out later. Draw the outline onto the plastic sheet using the permanent marker.
Cut out the stencil. Place the plastic sheet onto a worktable, scrap wood or a surface you wouldn't mind damaging. Using the pen knife, cut along the outline, making sure the blade is cutting completely through the plastic.
Place the stencil onto the surface you will be painting on. Use masking tape to secure the stencil onto the surface, taping the stencil's corners.
Prepare the airbrush and air compressor. Attach the airbrush to the air compressor hose by twisting the hose's nut until it is tight. Turn on the air compressor and wait for it to reach the desired PSI (pounds per square inch). For a stencil painting, it is recommended that you set the air pressure to somewhere between 10 and 30 PSI. If you have an external mix airbrush, pour violet airbrush paint into the paint cup. If you have an internal mix airbrush, fill a small canister with violet airbrush paint and screw it into the nozzle until it is secure.
Paint the background of the fish. Hold the airbrush 4 inches from the stencil. Gently press down on the airbrush trigger to let out compressed air from the nozzle. Pull back on the trigger to release paint. The farther you pull the trigger back, the more paint you add to the airbrush's spray. Paint the stencil by using even, horizontal strokes. Apply paint until the fish is completely filled with an even coat of paint.
Clean out the airbrush and paint cup or canister using water. Fill the canister or cup with blue airbrush paint.
Paint the scale details of the fish. Holding the airbrush 2 inches from the surface you are painting, paint vertical scalloped lines down the body of the fish. Carefully paint the fins and tail of the fish with an even, opaque coat.
Clean out the airbrush and paint cup or canister using water. Fill the canister or cup with silver airbrush paint.
Highlight the fish. Holding the airbrush 3 inches from the body of the fish, paint light silver spots inside of the scale details. These highlights are supposed to be fuzzy, so add only a little paint to the spray (only slightly pull back on the trigger). Cover the fish's fins and tail with a thin coat of silver, and then leave the paint and stencil to dry.
Remove the masking tape from the corners of the stencil. Carefully pull away the stencil.