I actually live-streamed (nearly) all 11.5 hours of development of the game, which you can check out here if you're interested (this was the stream day where I had most of the boring stuff figured out). As with most levels I create, I start with an idea of something I think might work, then play around in the game's sandbox. Sometimes I catch myself by surprise with what's possible! I started Saturday with only 3 levels made and developed the remaining 9 over the course of the 5.75 hour stream (along with lots of other improvements to the game engine). I was sharing builds as I went and was lucky to get feedback from those watching the stream.