There are a vast array of methods authors use to create the worlds, each one different from the other and suited to that particular person. There is no one, fool proof way. The only thing they all have in common is that building worlds takes time, a lot of time. If you’re aren’t willing to put the work in then the end result will most probably be an unconvincing, drab world that the reader can’t engage with. Detail is the key, if you don’t have detail then you don’t have a world.
Some authors use a preexisting, or ancient, civilisation as a template and then build from that, changing things to suit the kind of world they want to make, such as in Guy Gavriel Kay’s novel ‘Trigana’, which is based on the Roman Empire. Others focus on much smaller aspects first, a town or place, an institute like an…