There is no one method to become a trainer. In my experience so far, I have hardly seen any young professional articulating his ambition to become a trainer at the start of their career. Corporate training is typically a job people come to after they’ve worked for a while and gained considerable experience.
A good first step for professionals is to develop a technical or functional skill that they can build on and use as a way to transition over to a training job. Expertise in a field will also improve the credibility of a professional which is critical for the success of a corporate trainer.