To create sub-folder, navigate to desired (top) folder structure and click on New Folder button:
- Since this is sub-folder (belongs to your top folder) you can set any desired folder name (for example images or videos).
- Click on Create to finish new sub-folder creation.
Please note: top folders (first level within your /root folder) need to have unique folder name (top folder names are shared across all accounts, so choose something unique like mySiteName). More about top folders