I mentioned that I knew I liked Zettelkasten within the first 30 minutes. I think it might be important that when I sat down to try it, I had an idea I was excited to work on. It wasn’t a nice solid mathematical idea -- it was a fuzzy idea, one which had been burning in the back of my brain for a week or so, waiting to be born. It filled the fractal branches of a zettelkasten nicely, expanding in every direction.
abramdemski suggests starting with an idea you're interested in working on and fleshing out when you start your zettelkasten. This harkens back to Montessori teaching philosophies.