I’ve had the idea on the backburner for a while to make a UI-based idle-auto-battler mainly as a toy to mess with, but I have little clue how to start.
I want the main ‘gimmick’ for the game to be that the player has a toolbox menu to create different biomes, enemies, drops, weapons and accessories with crafting recipes, all with imported images from files, and a damage resist/vulnerability system.
I know that’s much farther down the road than starting on UI, so how would I start on that?
I recommend starting here:
Since just based on your post, I have no clue how much you already know.
Sounds interesting! I happen to really like idlers 
If you have little or no experience with Godot, I agree with the others that starting with a tutorial would be best.
After getting into the basics, Godot is thankfully very easy to build UI in. Personally I’d start writing down which features I want (like the toolbox menu, a map screen, accessory equip screen, …) and building the UI from that - a Godot scene for each screen in the game, buttons etc for input, classes for groups of objects like enemies / weapons / …
“create different biomes, … crafting recipes…”
Sounds like a idle game where you operate Minecraft servers…
