This approach/technique can be easily adapted for any of your favorite Cars characters depending on the colors you select. The car top is the same for any of the cars, it is just the personal details that change.
First things first, start with a match box cut using the Big Shot and Match Box die. Assemble per the normal instructions. For Lightening, use Real Red. For Mater, use Soft Suede or another suitable brown hue.
For the car top/cab, you need to cut a 2 x 2 1/2 inch piece of card stock. Score at 1/4", 3/4", 1 1/2" and 2 1/4". Crease each score so that you have two 1/4 inch tabs on each end. The tabs will be glued to the top of the matchbox to create the roof. This step is the same for both characters.
For McQueen's spoiler, you need a 1 x 2 inch piece of card stock. Score at 1/4 inch and fold such that the larger part is going up. Glue the 1/4 inch tab onto the car.
The best way to figure out how to add the details and pieces to create your characters is to study any pictures you have from the web, coloring books, stickers and other books. I think I've also watched the movie a few dozen times with Liam.
For Liam's party, I'll only be making Tow Mater and Lightening McQueen. However, someone at work has requested a set so I may try to make some of the other characters as well to fill out her set. Plus, I think I like the challenge.
Here are some more photos to help show the details:
God Bless and Happy Stampin'!