The brief summary on how to use the tilesets is that you have 2 options: set up the tiles directly on a tileset node or create a tile set by working with basic sprites on Node2D
Usually the second option could be easier to some extent, but it has some caveats.
For a hands on example I would suggest checking the isometric demo in the free templates the engine has online, it has a goblin on a dungeon icon (for that example you will have to change the tilesets to squares instead of diamonds)
I would also suggest to check some tutorials to help you get on track, because it may be difficult if you’re barely just starting on the engine (specially for doing the first option, since you have to tinker with a node’s resource that isn’t so friendly to handle first time)