Usually the first step to restoring or customizing a Hot Wheels is taking it apart. Hot wheels are permanently closed with a rivet(s).
The best way to get a good paint job on a die cast car is to get all the old paint off. The easiest way to do this is with a paint stripping chemical.
Getting a smooth, bright and shiny coat of paint on your customization is probably the most important step.
Of course the car has to be attached back onto the base, but also new axles or other custom parts that you have added.
Adding addition detail with decals is a great way to easy way to individuality and flare to your customization.