Sub Menu – Drop Down… How to?

Ok so this is probably a basic question but I cannot seem to wrap my brain around how to do this.. I am coding a custom theme and I need the sub menu to be displayed as a drop-down only when you mouse over the main menu item that contains the sub menu.. I tried to put in my sub menu items from appearance menus but it breaks the menu and page because it displays those (sub)menu items like it would a regular menu.. So my question is how do I get my sub menu to ‘hide’ until someone hovers over that main menu item with the sub pages and then have that display as the drop down?

*I’m sure I can get the drop down part but hiding until moused over is the part I cannot grasp..