go to the Bookmarks menu item, in the menu bar, and select "Show All Bookmarks"
Once all your bookmarks are displayed, go back to the Bookmarks menu and choose "Add Bookmark Folder"
Once it is created, you can put it wherever you want it.
How do I add sub folders under Safari bookmarks? For example, Finance is bookmarked. I want to add folders under that heading, e.g. Banks, investments, etc.