Ummmm, it's alot easier to do them tha it is to do a DIY on how to do them.
You need some big pics of the actual pixar cars themselves, and cut and paste the best fitting eyes and mouth, then free transform/distort(ctrl T) them to fit. Then it's just a case of matching the color of the donor car with the cut and pasted parts.... thats the hard part, and black cars are the hardest, surprisingly yellow was very easy
I use lumination/replace color/brightness/contrast and matching color tools on photoshop. Also alot of history/eraser and eye dropper/paint brush tools as well. It a bit of work to match the color, but playing around with different tools, you'll sort it out. Best thing to do is start with a white or light colored car and play around with different tools. Ctrl Z is your friend
Here's some more.