Flexbox Froggy is a fun interactive game to learn flexbox. Once I started I didn't want to stop. I think it took me about 45 minutes to complete all 24 levels. The 24 level was by far the hardest. The way it is laid out and taught makes it fun to learn. I really liked the way it explained different concepts at the start of different sections. It starts with justify-content, then goes to align-items. After a couple of steps it starts to have the user combine the 2 properties and use them together. The property I had some problems with at first was the order property. I had never used it before and I couldn't figure out how to write it. But once I figured it out it went pretty well. At the end, the final problem combined 4 properties into 1 situation. There were 8 properties to pick from. It was difficult and took me a bit, but after thinking about it, I was able to figure it out and complete the 24 levels!
With a quick google search, I found Josh Comeau's web site that is an interactive guide to flexbox. It covers all the basic flex properties like: flex-direction, justify-content, and align-items. It also gave a lot of interactive examples where you could click different properties that would change the layout of the rows and columns to show the behavior of specific properties. To some people, me included, this resource is very beneficial. Being able to see how the different properties act instead of just reading about them is a very useful tool!
Like Josh Comeau's website, Mark Hendriksen has an interactive flexbox tool website as well. But, unlike Josh Comeau's website, Mark's tool allow's you to experiment with various flexbox properties and see how they all act in real time. The tool visually shows how changes affect the layout which would help the users better understand the concepts with hands on experience. The tool allows for use with properties like flex-direction, justify-content, align-items, flex-grow, order and align-self.
With just a quick google search, there are a ton of websites that give information and help any user learn flex. I find that using the interactive websites is an easier way to understand how the properties work. My favorite was Flexbox Froggy. It was a fun interactive game and at times really made me think. I definitely bookmarked these 3 sources on my computer too look back on in the future.