The demo used one hiding rule and two reveal states. In the supplied navigation, the Stalls button and two working in-page links are already present; only style.css needs editing. The menu is positioned and its links are styled for you.
.submenu to display: none so the links start hidden..menu-item:hover .submenu, set display: block to reveal the links on pointer hover..menu-item:focus-within .submenu, set display: block so focusing the Stalls button or either link keeps the menu open for keyboard navigation.Checkpoint: Preview initially shows just the Stalls button. Hover reveals the links; tab to the button and then into the submenu to check the keyboard path. CSS-only menus have limitations on touch and do not offer Escape-to-close; a full disclosure widget needs JavaScript later.
Keep the stylesheet linked from the document head.
Change one declaration at a time, then use Run Preview to inspect the result.