World building is one of the most enjoyable parts for me. Not just the world building, but the entire plotting of a story. I use Scrivener lately to help me compile all of my thoughts in one place. I usually compile a few folders with documents within them to cover off the planning. For example, for my latest children's fantasy my planning folder consists of:
Story Structure - which includes all book plots, story/series arcs, series plants and timelines
World - which will include information about locations such as schools, governments, cities/countries. It will also include objects in the world, species (my story has different races) and spells (also a magic based story).
Characters - I go over, in full detail, the protagonists, antagonists, secondary characters, families and minor characters
Histories - If it's an extensive fantasy world, I usually write up some short stories to help with the backstory. My current book in progress features humans, elves, dwarves and goblins. Over the years they've been in conflicts, so I write up their histories as a short story.
I might go into more detail than most, but I find that creating this level of granular level information allows me to truly flesh out my story. And so, when it comes to writing, I have far less to adjust in the end.
Whenever I start a new story, I write down what these folders and text documents would be and create them. Then during the planning stage I begin to fill them in.