Have a look at this article on structure and naming before creating a folder:
How to create a Folder:
Right click in your Flipbook tree (Alternatively left click the tree dots found on the right or use the New folder button).
In the drop-down, select "Create Folder".
Name your Folder.
But how do I move my Flipbook you may ask :D Click here for an article on that.
Remember, as mentioned in the top, moving Flipbooks will change the structure of your Flipbook URLs.