Tutorial: Side Vent (airbrushing from scratch!)
This tutorial shows you how to create scratch side vents on cars. Airbrushing is the main part of creating this vent, so make sure you are up to scratch with doing this.
       
 Side Vents
 
Difficulty - ****

You first need to plan out your vent.. I usually do this by making a new layer.. and then planning it out with the line tool. Pick some bright colour that can be easily seen. Bright green is usually the choice of the professionals.. lol. In this case, I'm gonna make it right up to the doorline.

 

Create another layer behind these green lines and make a selection like so using the polygon lasso tool. Select a lighter colour from the bodywork using the eye dropper tool, and using the airbrush, gently fill it in. This acts like a highlight and makes it look like the bottom of the vent.

 
Do exactly the same as before, but select the vent part and fill it with the colour black.

 

 
Next you want to add some very small amounts of shadowing to the top of the vent line. So again make a selection like on the left using the lasso tool and slowly airbrush in some black shadow. If you select a large sized brush (about 100pixels) and then paint a fair bit away from the actual line, the "overspray" will only show up - making a great shadow.  
Now, hit Ctrl+D to remove any un needed selections, and then delete the layer with the green lines on.

The vent is now complete.