Setup

The first time you run the Character Editor, it will ask where you have downloaded the GitLab source to. If you are reading this, that means you probably figured that step out already, but if not, navigate to the root level of your game folder where you extracted the Git respository download (i.e. usually named spnati.gitlab.io by default, the one containing the js, css, opponents folders, etc.). The editor needs to know this information so it knows where to read character information from, among other things.

Creating a New Character

Once you've finished setup, you are ready to create a new character. Click File > New and choose a name for your character's folder. Usually this is your character's first name in lowercase unless that is already taken. Click Create.
Upon creating your character, it'll inform you of the name of your folder in case it didn't like what you picked.
At this point, you'll be taken to your character's workspace with the metadata tab available. If you save now (File > Save or Ctrl+S), your new character will now be available to play with in the game (offline). To test, open index.html in Firefox (Chrome and Edge do not work for the offline game). Your character should appear at the very end of the character selection screen with a blank box.