Well I not the worlds best customizer nor do I claim to be. But I do have some good tips to pass along to you the masses. All of this is from my own experiences and your mileage may vary. There is no one way of doing a custom, experimentation is the key. Don't just do repaints, try to make a convertible or a hybrid. Try making flames or checkers, detail your customs. Make engines really look like engines with plug wires and hoses. 

Anything your imagination can come up with you will be able to do to your Hot Wheels, it just takes time to learn all the basics and to perfect the harder stuff and to challenge yourself to a higher goal. Even the best are still learning. Look at someone's customs to get ideas, try to copy it. You will find it rewarding to push yourself in a direction you never went before!

Last- talk to others, join a customizing group or a Hot Wheels club-all these will be beneficial to your overall experience in the world of custom Hot Wheels.