Rachel Andrew's look at Flexbox and CSS Grid and her Grid By Example (THE site to follow)

Short beginner Flexbox: in 3 minutes; in animated GIFs

Flexbox from CSS Tricks and Sitepoint course part 2; Flexbox patterns (might be useful for assignments); Flexbox Froggy (can you get the last one?); Wes Bos's What the Flexbox?

OK—the best Flexbox in 5 Minutes resource. Note how all main properties are in the Code samples. And here's my version of the Flexbox Patterns card group. This is the Everything flexbox post (99 pages)

Flexbox cheat sheet (from Joni Balogna !; PDF doesn't print well)

Grid Garden, CSS Grid Starter page (links)