100% trial and error. There are some schools that will teach you what to do, but if you do not have the basics you will be lost when you show up. If you really want to learn get a job at a shop or big box store as a sales person, but let them know you are interested in installing as well. Chances are they are more likely to give a current employee a chance in the bay when an opening comes up that has interest than someone from outside. All of my current installers at CC were at one time sales people. I myself started by tearing apart my personal car and friends cars when i was 16, got a job during college selling, then moved to installing and am now the sales and installation manager. The other guy i learned with during highschool on our own cars is now the lead installer at a very highend shop in atlanta.