There are 2 ways you can start your game with this. A lot of people just... used that game as their base game, like using it as a template. This way it saves a lot of time but I won't recommend this method if you don't quite understand the concept of eventing in RPG Maker.
The way I recommend is to start a new project, which all you need to do is copy the whole plugin folder from Action Combat game folder to your project. Basically, it's just the things you normally do when you download a plugin. After you've enabled the plugins, you're ready to make a game. Here are the tutorials and lessons that you can try.
https://rpgmakeractioncombatmanual.blogspot.com/
You can also combine both methods. Create a new project but still use the Action Combat game to see how I set things up and try to mimic my method, just to understand things more easily. Otherwise, you can pretty much use this plugin however you want.