Here is the official tutorial video series to help you get started:
https://youtube.com/playlist?list=PLZj2AIMJmKKnJYVMIY-cLbBDz3Ts-GrAi

The Mod Tools for Epic Fantasy Battle Simulator are provided as a separate app on Steam, in the TOOLS section.

In your Steam client, go to the library page, and enable the tools listing like so:


Then find the Epic Fantasy Battle Simulator Modding Kit, and install it:


In case the Modding Kit doesn't appear in the list, even with the tools filter on, exit and relaunch your Steam client.

Click the Launch button, or manually open this folder:
C:\Program Files (x86)\Steam\steamapps\common\Epic Fantasy Battle Simulator Modding Kit

You'll find a PDF documentation file in the folder, that contains everything you need to know to mod and upload to the Steam Workshop.