There are plenty of helpful resources online to help you with all manner of web design projects, including WordPress tutorials and free web fonts. But in this article we're focusing in on articles and videos to learn more about Flexbox and PostCSS. Here are some we like:
A great introduction by Drew Minns covering what PostCSS is, its history, and detailed setup instructions for how to include PostCSS in your workflow.
Chris Coyier wrote this article explaining the basics of CSS3 Flexbox and how to use all its properties and values. This can serve as a helpful cheatsheet.
Kezz Bracey has put together a PostCSS Deep Dive series on Envato Tuts+, containing multiple informative and actionable articles. This one helps you build your own PostCSS plugin from scratch.
PostCSS.parts lists the plugins available in PostCSS categorised by function. This can be a great starting point to evaluate PostCSS for your development workflow.
This is the documentation from the makers of PostCSS. This offers a boilerplate, guidelines and tools for writing your own PostCSS plugin.