How do I make a library?

Hi again out there.
This is the story: I have made a new scene in Toon Boom studio, A simple house with some mountains in the background and a pathway and a group of trees in the foreground. The group of trees is made by drawing ONE tre, and dublicate it as may times that I want. So far so good. BUT, it would be nice if I also could save the tree in a library so I dont have to draw it all over again if I need some trees in another scene in the future. So the question is: How do I save an ellement (like for example a tree) in a library?
Best regards