This small guide covers how to test changes to the data with Optolith.
You can (and probably should) check if what you worked on, first of all, is valid and, second, does what you expect.
Simply run
npm testin the terminal in the repository folder. It will output if files have errors and what these errors are.
This will do structural testing, i.e. validate the files using the JSON Schemes that already help you catch errors in the editor, as well as semantic testing, which includes checking if referenced entries actually exist and other checks that cannot be done using simple JSON Schema validation.
To do that, you'll need a prerelease version. You'll get a link either on the Discord server or in an e-mail. If you don't have a prerelease installed yet, install it now. It will be installed separately from your stable installation and it will also not affect your characters in your stable, so you're pretty safe to do that.
Go to the installation folder and then in the resources/app.asar.unpacked/app/Database. You'll notice there are a lot of folders that are in the Data folder of the project as well. Copy the files and folders from the Data folder into resources/app.asar.unpacked/app/Database so they match the existing folder structure. Then, (re)start the Optolith prerelease. You should now see your changes and can try them out. If the source files are invalid, there'll be an alert and you won't see the characters list.