When shrink div box, tutorial?

[TechnoBear]

Not a tutorial, but these recent threads may help:

From 2 columns to one

For my responsive home page, I would like to have a 2-column design for the desktop, and for mobile just have one column. On the desktop, there would be a narrow right column that looks similar to where Google news has its highlighted articles. But for mobile, I need those highlighted articles to collapse down into my main single-column bucket. What is the best way to achieve this in a responsive manner?

Changing column layout per device

For mobile devices, I would like my layout to be one column, and for desktops I need a main column and a smaller right column. Still trying to get my heads around RWD. Could this be accomplished by using display: table and having a mobile media query with just one column, and a desktop media query with two columns? Or is there a better approach? (I read up some on flexbox, but that seems too advanced for a RWD newbie like me, so I was hoping for a n easier approach.)