Access your own skills. If you seem to know more than most of the mechanics you know, you might be in a position to offer a service to those who could benefit from your knowledge.
Go to school and receive some accreditation. If you want to get hired on to a big company or at a tech school, you will have to show some academic training in your service background.
Watch for openings at local community colleges. Teaching at community colleges is a great way for professional mechanics to get started in this profession. These schools hire a lot of adjunct faculty. They typically promote their courses with bios of various teachers.
Talk to automotive manufacturers. Most of them hire auto repair professionals to sell their equipment or offer after-market training to customers. This business can be more lucrative for a mechanic trainer.
Start your own business. You could begin small by running an ad on craigslist.com fro your area and then take on individual students. Many folks could benefit from learning how to service and repair their own vehicles.