I then changed the name and display name, and set it to create Lava surfaces instead of Water surfaces because I'm a bad person. Since I was just creating a test skill, I opened up Divinity: Original Sin 2's Rain spreadsheet, and copied all the data from the basic Water spell into my Rain spreadsheet. This will create a big blank spreadsheet, which is a bit daunting. Under your mod's dropdown, hit the + symbol next to 'Skill Data' and select the type of skill you want to add. Anything you add there will be added to the game (or modify something that already exists) if your mod is active. You'll also see a dropdown for your mod with nothing in it. Here, you can see (but not directly modify) all of Divinity: Original Sin 2's items and skills.
In the upper-right hand side of the menu bar, you'll see a bunch of different icons for opening editors.