Well, with practice I guess! For each step of the developpement process I knew what to do and how much time I have. I can give you some time-stamp if you want:
DAY 1: (for me the game-jam started at 1PM)
- Brainstorming the idea (~2h)
- Making a simple demo (~3-5h)
DAY 2:
- First graphics assets (the first few boxes, player character)
- Overall game design (the map, the UI)
- Adding some more features if time’s allow it (undo button for exemple)
- HAVING A WORKING LEVEL IS A MUST HERE (for this game at least)
DAY 3:
- Finishing the few last bugs
- Upgrading/Making the tilemap
- Music and sound design
- Level design. If you have a good workflow this should be really fast, you just copy-paste your test scene and change the prefabs)
- Playtesting (always keep around 2-3h to test / fix bugs / add some small features)