Twenty Twelve Theme: How do I make the menu two level

I love everything about this theme (and managed to find a workaround for the IE7 incompatibility).

The only thing I want to change is to make the menu two level of hierarchy.

If I create a child theme, what do I need to change on the CSS to make the menu multilevel menu (two deep). I see lots of code when I googled for it – but I am guessing there is a ‘proper’ way to do it.