Resource 1

CSS Tricks has a great guide to Flexbox on their site that explains everything you need to know about using flex. Their site is east to navigate and covers all of the properties, terminology, and tricks to make using flexbox a breeze. There is a table of contents making it easy to select the topic you are working on and check browser support. They have example code available for you to play around with as well as in-depth explanations as to what the code does.

Resource 2

W3Schools is a great resource for learning flexbox. Their navigation is on the sidebar rather than a table of contents so it seems more limited in information at first glance. This is a great resource for those just starting out in flex as it goes through the basics without too much extra code or information that can make flex seem overwhelming. They also do a great job of keeping their site up-to-date. They break their flex navigation down by the basics, containers, items, and responsive.

Resource 3

MDN has a wonderful section on flexbox that is in-depth and has comparisons with other layout methods. They also utilize sidebar navigation but with the added right sidebar to navigate through articles to sections. I, personally, really enjoy this navigation style as it makes finding exactly what you are looking for easy to find without much scrolling. They cover everything you could need to know about flexbox in a thorough and easy-to-navigate site. This is my personal favorite choice for resource guides for coding.

Summary

There are many different resources available for learning flex. It is important to make sure you are looking to a reliable site when working on any site as you need to make sure you are staying up-to-date, following accessibility guidelines, and rules set forth by the W3C.