The best way to become a musical instrument repairer is to work under someone who currently holds such a position and learn as much as possible about the craft. There are many fields of musical instrument repair, so it may be wise to choose one or two specific instruments that you will like to become an expert at repairing. Another way to become a musical instrument repairer is to take courses that teach instrument repair; such courses will often focus on one instrument or family of instruments rather than the entire spectrum of musical instruments.
Once you have decided what specific instruments you intend to repair, you can take steps to become a musical instrument repairer by finding out if repair classes are offered in your area. This may mean researching local community colleges or universities, or it may mean visiting local music stores to see if the repair department offers classes or guidance. If such classes exist, enroll in them and gauge your interest and abilities in the repair realm. If such classes are not offered, it is time to consider other options to become a musical instrument repairer, such as an apprenticeship.
Ask local musical instrument repairmen if they will take you on as an apprentice who observes and participates in the repair process. This is likely to be an unpaid position, but it will give you valuable knowledge and experience that can help you become a musical instrument repairer. Be sure to take careful notes, and try hands-on techniques as often as possible. This apprenticeship may last several years, as most instrument repair involves specific techniques that can be difficult to master. You may have to work a full time job and do the apprenticeship on the side, or you may work full time with the repairman to learn the skills and become an asset to that particular company.
If you want to become a musical instrument repairer who deals with electronic musical equipment, a background or education in electronics is a very good start. Learn the basics about electronics, including theory, practice with soldering, and repairing other types of electronics. This experience will help you understand how electronic instruments work, how to troubleshoot them, diagnose them, and repair them. Amplifier repair is a good industry to get into; this is a complex task, but is one that is in fairly high demand. If you have the skills and knowledge to do amplifier repair, you may have a better chance at becoming a successful instrument repairer.