If you go to Settings > filtered tags, there is a box into which a user can put tags that they want to "muffle". Any tag you have muffled is upgraded to a content warning for you specifically. For example, I've got moderate mycophobia so I've muffled the #mushroom tag. When I see a post tagged #mushroom, it looks like this:
Because tag muffles exist, I can avoid seeing mushrooms without everyone else who doesn't mind them at all having to click through a content warning to see them. It's a really good system specifically for stuff that's totally fine for most people, but unpleasant for a minority. It's also useful for people who are particularly spoiler-sensitive, tired of a particular kind of discourse, and so on.
As to how you post things for them, the main thing is to just add tags to your post that cover its major subjects. I personally try to always tag things like food, alcohol, drugs, politics, common phobias like snakes and spiders, and any piece of media I discuss in the post.
