Create an "addons" folder in your project and drag the folder for the entire plugin into that folder. Then, in Project -> Project Settings, find the Plugins tab at the top and click to Activate the one shown.
As far as interacting and stuff, I think that's a bit too subjective of a question; it depends on how you want to setup your code, it depends on how things are setup already, and so on. What I'd recommend is if the player clicks on a particular tile, you go to your tilemap, see if that tile is an object which can be interacted with (like a tree) and if it can, then you automatically navigate the player to one of the tiles around it - depending on your animation for it. So for instance if the player has to swing an axe, and that axe will swing from behind, down, and then to the right so it looks like it's "hitting" the tree, then you'd want the player to be in the cell to the left of the tree. We can't really explain to you here how to code an entire game though, so you'll have to figure out some of this on your own. I recommend using the plugin I made for navigation, but use it to learn how to handle navigation and learn on your own - it's more important to learn how to do stuff on your own and be self-sufficient.