Unzip it somewhere sensible, and in your stage file go to File > Append and
select both of the objects. Do not rename them, the game uses the name to make
the countdown timer work automatically.
A rip of the native stage Bisanne, which you can use to understand approximately how the objects should look. Note that this is was produced by the importer, which does not yield an exportable stage. Nor is this file “perfectly structured” because all geometry is pre-merged, the ground collision mesh is separate to the ground visual mesh, etc - so take it with a grain of salt and ask questions in the Discord if you’re confused about something.
As above, but a native gravel stage.