CSS-Tricks
CSS-Tricks has a good guide to Flexbox. It shows detailed explanations of all the properties and some interactive examples to help see how it all works. A full 11 sections with some additional sources to consider.
W3Schools
There are quite a few tutorials on W3Schools. They also have an editor to try out flexbox properties yourself in the browser. All the properties you could use are here with images showing examples for things like responsive design.
MDN Web Docs
MDN has plenty of information. They explain how Flexbox handles one-dimensional layouts in comparison to other layout models like CSS Grid. Also how Flexbox has replaced things like float and positioning. While they still work they are limiting in comparison. Images and code provided to explain the properties.
Summary
These three sites are the ones I most visit when trying to figure out CSS issues. They have resources for both beginners and experienced developers which is very helpful. Each has it's own way of describing how Flex works providing lots of examples using code and images.