I'm just a newbie and still figuring out how to do it...
First of all, I rule out industries that I feel will never be in my circle of competence (because I don't have interest, too confusing, can't wrap my mind around it, etc).
I usually just read through company presentations (several competitors in the same industry), as well as the industry overview sections of 10K filings. I also find that I learn a lot from reading long-form magazine or newspaper or blog articles (I'm talking about general, mainstream, ones). You won't necessarily have such articles for all industries but when you do run into them, they are great. Some people also get a lot of value from reading trade journals/websites but I find them hard to understand and you need follow them/read them for a long time (many years) before you learn much from them.
If one has access to them, brokerage reports (usually initial one when they initiatiate coverage, or industry review) can be really good. But most people don't have access to them. I get a few free ones through my discount brokerage but they are a hit and a miss since free selection is very limited.
Finally, there may be books on the industry. Often, you won't find an investment-type book on the industry but you may learn qute a bit after reading the history or biography of some character.