How to make markup with flexbox?

Question

css

Hello! I am learning flexbox, but so far I have not been able to make a vertex of this kind - at the top is a navigation strip, on the left is a sidebar with a fixed width and a rubber content. Point me in the right direction, please!
codepen.io/anon/pen/avagmL

Answer 1

codepen.io/anon/pen/rOZEbJ

Answer 2

It seems to me that it’s better for you to look at live examples, especially from Vadim Makeev himself, a recording of 2013, at that time prefixes were needed, but now everything is much simpler, well, he shows the basic principles very well, I highly advise.
https://vimeo.com/67011034