Instructions
Place the game disc in your video game console and wait for the flash (logo) screen to give way to the main menu. Then select the Career Mode option.
Select Safe House from the list of options and you will be transported to your garage/safe house. From here choose a car from the list of options that you would like to customize. You can't build a car from scratch but you can change almost every aspect of a car you already have.
Select the option you would like from the customization bar located just under your car. You can choose the Performance option, Interiors or the paint job. Once in these menus scroll through the various options until you come to the look you would like for your car and select it. Then save your changes and close out of the screen and you will be back to the garage.
Select the Autosculpt option from the customization bar and then select the area of the car you would like to change from the boxes on the bottom of the screen.
Use the arrows at either end of the sculpt meter (located in the top left corner of the screen) to adjust and modify the look of the car part until you are happy with the result.
Select the Save option and close out of the screen. You have now built and customized your own car.