Flexbox Resource 1

Here is a complete guide to the css flexbox layout, as well as a lot of the properties for the parent flex element, as well as the individual flex item inside a flex container.

CSS Tricks: A Complete Guide to Flexbox

Flexbox Resource 2

Of course, the mdn web docs has a lot of reliable resources in regards to the flexbox layout, including information on how the flex layout compares to other layout methods, how to align items in a flex container, how to control individual flex items, and more.

Mozilla Developer Network: Flexbox Concepts

Flexbox Resource 3

Lastly, www.freecodecamp.org provides a lot of information on how flexbox works, a complete handbook as they, say, which defines what flexbox is, it goes through the properties of flex, as well as using several diagrams to show what happens visually with certain flex properties.

CSS Flexbox Handbook, by Freecodecamp.org

Summary

Overall, a quick search for guides on the CSS Flexbox will yield a variety of resources that can offer tutorials and in-depth guides to building a flexbox layout successfully, as well as managing said items in the flex container.