creating responsive side menus

hi. i found this blog post:

https://premium.wpmudev.org/blog/how-to-add-side-menus-to-a-wordpress-theme/

this looks like pretty much exactly what i need, except that i want the side menu/link to appear only when on mobile devices (or in narrow windows). otherwise, i want the normal top menu that i have now.

how would i install this to make it responsive? is it simply a matter of setting up special css? if so, what would the css be?

thanks!