You notice a community has gotten popular, so move in and spend all your time posting and building your cache so people respect you - but only talk about politics superficially - never reveal your actual political views - save that for later.
Get voted in as a mod at the first opportunity. This will be easy since everyone has seen your username posting walls of text, and thinks you're dedicated as fuck to the community.
Now become so unbelievably toxic as a mod that all the other mods leave and you can invite your friends to replace them.
Get to work banning anime, cat ears, ponies, Rojava and the word "dumb" because it's evidence of brocialism. Start purging all the actual leftists with bans and thread deletions.
Once you've totally destroying the sub, you can start looking for the next interesting community to begin hijacking.