Here is a timelapse video I made of building a level in my game Mouse Dreams. It’s sped up (otherwise it would be very long and boring), and this level was already designed on paper before I started building it in the Unity editor.
You can see how the layout was tweaked and improved as I progressed. Designs on paper sometimes don’t work as you’d hoped, and you need to move things around to get it just right, but the basic ideas of the level remain intact.