Very clever puzzles, though admittedly lvl 11 was a little too punishing for me, what with the long setup time. Still, well done!
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For me, it started with the theme which helped me make the mechanics. Then I made a list of interactions between each object and drew a bunch of levels that would introduce the mechanics and how the player can interact with objects. All on paper! Then start coming up with challenging levels that use those mechanics in combination with other mechanics.
Currently the only objects are walls, boxes, hearts, humans, ghosts and Ghostman in the game. I had a bunch of other mechanics I had thought of but slimmed down in order to make creating levels more streamlined. What I learnt from this game jam is that limiting what the player is able to do means you can tailor a puzzle to what they can do.
There's a couple of specific methods like reverse designing, where you set the goal first and then find ways to stop the player from getting to the goal. Designing a single level around a specific theme/concept/name/image. Pathway design where you start by placing the player and create a specific route you want them to take. And many more that I'm sure other game jammers here would be more familiar with.
But if you asked me IRL I'd say coffee and free time haha.