The level editor can be accessed and used at the following URL: Ash47.github.io
You need an existing "They Are Billions" save game, so fire the game up, and create one if you haven't already.
After that, load up the website above and press the "Load Save File" button, and then browse to "Documents" / "My Games" / "They Are Billions" / "Saves" and then select your ".zxsav" file.
This will load the editor. After this, you can edit the map using the tools provided, and then save it using the buttons at the top. You need to save both a ".zxsav" and ".zxcheck" file.
Here are some Dev VLogs which explain how to use the level editor:
