The term Homebrew originates in the alcoholic beverage industry, referring to people who brew their own beverages at home. The term has since grown substantially in scope, but for /tg/, "homebrew" usually refers to house rules, settings, stories, or even entire games made up by bored board browsers.
How to Homebrew:
So, you've decided to try your hand at building something off of an existing system, like, say, D&D. Alright. First step: Look real hard at the rules, similar classes, or other things like that. Alright, you understand what the framework is. Now, how offshooty is it going to be? A new weapon with one or two special rules? A special city? A new RACE? Make sure to make it semi reasonable. Then toss it over to a friend after proofreading any rules or lore. They might catch missed mistakes. Congrats. Add a little polish and you're good. Emperor save us all.