By this point, 22 community members created 34 custom levels using the Beatmap Editor app.
To learn how to download and install custom maps (levels), join a community-driven Discord server for AGAINST mapping and modding.
Everyone who owns AGAINST on Steam has 'AGAINST Beatmap Editor' in their library. If you can't find it, make sure that your selected search filter allows applications.
If you're feeling creative - make your own maps, there is an in-app tutorial and a community willing to help ;)