The real answer to the question on how to become a service technician for scuba equipment is really more complex than one would think. (For background, I hold technician credentials from virtually every major brand of scuba equipment and have been servicing equipment professionally for over 20 years.)
Every manufacturer has some form of training for technicians and some are much better than others. Virtually all of them require either employment by or sponsorship by a LDS to get that training.
The quirky part of this is that while all scuba equipment shares common technical functions, the forms are sometimes quite different. This means special tools and special procedures of dis-assembly, assembly and testing. It is this part that "separates the men from the boys" regarding expert service of scuba gear.
If divers only used the newest and most common pieces of equipment, life as a technician would be easier (and it would be easier to train someone to be a service tech), However, there are divers using some of everything under the sun and eventually that gear comes in for service or repair.
It takes, in a store that does a LOT of service, two or three years to become exposed to enough of the gear that isn't mainstream to feel confident that you know something about "gear" at large. It takes an number of failed attempts to resurrect some out-of-date piece of gear to get to th point that you know when NOT to try to fix something.
In our facility, when we train a person for tech work, in addition to the theory and mechanics of a given piece of equipment, they are expected to serve an apprenticeship under one of our professional technicians. Only after thet have received our training, and our approval, do we sponsor them into a manufacturer's service program. (Thinking here is that if they don't even know how to use tools, or have some kind of knowledge base to build on, then a manufacturer's technician course will be wasted on them.)
It is true that Scuba equipment is not extremely sophisticated or difficult to work on. It is true that it takes a lot of special tools and procedures to work on Scuba gear correctly. And, it is true that one has to be very patient and consciencious in working on life support equipment.
The art in equipment repair and service is not in slinging wrenches and installing parts, it is in the final setup and tuning - knowing how to set a regulator for peak performance and maximum time span of dependable performance is an acquired talent, and having the right daignostic equipment that is sensitive enogh to really tell you what is going on in the regulator. Very little of this can be taught in a one or two day manufacturer's seminar.
My best advice is to find a true professional technician and beg, borrow, steal his time and knowledge and work at his side for whatever amount of time it takes to become proficient at service and repair, and never let someone leave with a piece of gear that you wouldn't dive with yourself.