Find a school near you and volunteer. Meanwhile, go to college and get a degree in animal behavior to make yourself more marketable. Volunteer at animal shelters rehabing dogs to make them adoptable. The more dogs you work with the more your skills will grow. Apprentice with a local dog trainer (guide or otherwise) to learn more dog training skills.
Each dog is an individual. The key to a good trainer isn't as much knowing theory as it is knowing dogs and how they think and being able to communicate effectively with them. Theory on guide dog specific training must be learned from the guide dog school, but the basic foundation of dog training theory is learned by working with many different dogs and needs to be a life-long commitment on your part. Most guide dog programs, such as the Seeing Eye, require a a college degree and an apprenticeship on top of basic dog training skills. For the Seeing Eye, that apprenticeship is three years.
If you pull together the skills needed, the school you volunteer and apprentice with may either hire you or give you a letter of recommendation to work for or apprentice with another school.