Build a level from snap-together sections, then export it as a level file for src/data/levels/.
Import a bundled level or a levelNN.js file. Its terrain is preserved exactly (section editing is locked, since it wasn't built from sections), and you can adjust its details and smashables, then re-export it.
Add sections to build a new track.
Pick the placeable kind (Smashables or Bonus items) + its type, then click anywhere on the Preview map below to drop it there. Click a placed item (on the map or in the list) to delete it. Bonus items are float-only; smashables can float or fall to the ground.