Double-click on the Matter Type with the document folders you wish to modify and then select the Default Document Folders tab.
Select Add a Folder, enter a name, and then select Create. You can add as many folders as you need.
You can create sub-folders by creating the folder and dragging under the appropriate folder.
You can also right click to add a sub-folder.
You can create matter-specific folders by right-clicking the source folder and clicking Add folder