Add a folder to your Templates folder

To add a subfolder to your Template Folder choose Templates in your sidebar menu (1).  Then in the template folder click on +New Folder on the bottom of the default subfolder list (2).

Then in the modal window that pops up enter what you wish to name the folder(1)  and then click OK (2). 

Once done you will see your subfolder(2) in the Custom subfolder section under the default subfolders.