I was just asked similar question, so let me share with you, what I told my friend, and many others who have asked me…

I do not have a set or rules and method I could write out. However what I can advise, is that you must spend a lot of time looking through car pictures. You will want to match cars with pictures of similar angle, shadow and highlights, from which parts between pictures can inter-change. Colour can be tricky. But that can always be made into grey-scale, then colourised again when the car is mostly done. Most importantly, you need to practice again and again and again. The more you do, the more you are familiar with all the filters and widgets in Photoshop that will help make your chop more realistic. And as I learnt, be patience, and have confidence…

Good day to you.

You have a great weekend Raju!