MDN
The Mozilla Developer Network is a trusted source for coding information. They define what the flexbox model is and clearly explain the various properties and values. They provide multiple examples showing a snippet of code with an illustration of how the code will render in a browser. They start with the most basic concepts and continue to build which is good for a beginner. After the explanation, they provide a link to a page called Test your skills which provides interactive tasks to be able to work with the various concepts to get some actual experience using flexbox. They also list additional sources they recommend to further explore the topic.
➡️ MDN
Stephanie Walter's Design Blog
The purpose here was not to teach or explain flexbox but to act as a resource to point you to other sites which explain and show how flexbox works in a fun way. Her goal in compiling this list was to make a potentially complex topic less intimidating. The resources she lists include games, cheatsheets, animations and videos.
➡️ Stephanie Walter's Blog
freeCodeCamp
It would seem their ‹h1› says it all: "Learn CSS Flexbox in This Crash Course". There is an embedded video to their YouTube channel tutorial and also a link to the video out on their YouTube channel. A transcript of the video follows but isn't as helpful as the video. PDF cheatsheets were mentioned but I felt it was presented poorly as the PDFs are not accessible from this site. After poking around, I found them by clicking the provided external link to YouTube and then down in the description box of the video are the links to the cheatsheets. However, seeing someone actually demonstrating the code can make it easier to understand than just reading about it so I feel that is a positive feature.
➡️ freeCodeCamp
Summary
My first thoughts on what resources to use were the tried and true standbys of w3schools and MDN. I remembered there were tools such as the games to help learn flexbox so searched out those. freeCodeCamp came up as a recommendation as I was searching. Once I went to the their YouTube link, all sorts of recommendations for tutorials popped up. Apparently one can learn all about flexbox in anywhere from 15 to 43 minutes! But that just emphasized that there are many resources out there. It is hard to know how reliable some of those may be however. Therefore it would be best to use some caution so as not to pick up any bad habits or tips which could cause problems on one's site.