Navigation Menu Drop-Down Box

I’m looking for help creating a “custom” drop down menu “box” from my main navigation. What I’m looking for is something similar to the “Products” drop down on this website: https://www.midwestgrip.com/

So basically just a box that drops down, where I’m able to have titles of categories with links to pages below. What’s the best way to do this? I am just modifying the Twenty Twelve theme.

Any help would be greatly appreciated! Thank you