OK, very helpful video. I managed to create the sub-menu with no difficulties.
I now have 2 questions to finish what I'm trying to do:
1. What's linked to the main menu is a category. In addition, I linked each of the 3 content items in that category, to form a sub-menu.
how do I define it so that only one of the content items show when I choose from the main menu (and the other content items are shown in the sub-menu on the right)? now, all 3 items are displayed.
2. How do I make the new sub-menu's headline not appear? (since it's a sub-menu, I don't know where it's defined)