The library works with two basic classes:
World class is where all the interaction and gameplay takes place - it's where all the
Actors live, and for most games one
World is enough.
Actor class is where all the per-item code goes: you might have a player
Actor, one for an item the player can pick up, and
maybe a few ones for obstacles.
First, go to the Releases tab and download the .zip file, and unzip it to wherever you want to keep the library. To install the library, simply open
Green-<version>.zip/library/ and put
Green.jar in your Processing libraries folder. Processing's
is located in
<sketchbookLocation>/libraries, and it's sketchbook location can be found in the editor under
File > Preferences....
From there, select
Sketch > Import Library... in the editor, and Green should show up for selection.