Wes Bos - CSS Grid

Free walkthrough of main concepts and example projects

https://cssgrid.io/

Wes Bos - What the Flexbox?!

Free walkthrough of main concepts and example projects

https://flexbox.io/

CSSTricks - A Complete Guide to Grid

Comprehensive overview

https://css-tricks.com/snippets/css/complete-guide-grid/

CSSTricks - A Complete Guide to Flexbox

Comprehensive overview

https://css-tricks.com/snippets/css/a-guide-to-flexbox/

Grid Garden

A game for learning the Grid

https://cssgridgarden.com/

Flexbox Froggy

Game to learn the techniques

https://flexboxfroggy.com/