Purchase or borrow any and all service manuals for the vehicles you would like to customize. Service manuals will provide you with the requisite mechanical information for the cars you want to customize so that you don't remove or replace a part essential to the car's operation. Many service manuals for newer cars are now available on car company corporate websites.
Purchase or borrow customization tutorial guidebooks related to a specific type of customization.
Watch tutorial videos online through free video services such as YouTube, Google Videos or eHow. Though there are pay-per-view sites with specific tutorial videos, many similar videos are produced by private mechanics and car customizers.
Practice customization techniques on junk or practice cars before moving to any customizations of car you hope to drive or sell.