Install the create-react-app CLI

npm install -g create-react-app

Create the app with create-react-app CLI

# create with create-react-app PROJECT_NAME
create-react-app postcss-example
# change into the postcss-example directory
cd postcss-example
# eject the app to expose dependancies and configuration files
npm run eject

Hook up CSS modules

// change: webpack.config.dev.js
{
   test: /\.css$/,
   include: srcPath,
   loader: 'style!css!postcss'
},
// to:
{
  test: /\.css$/,
  include: srcPath,
  loader: 'style!css?modules&importLoaders=1&localIdentName=[name]__[local]___[hash:base64:5]&camelCase!postcss'
},

Breaking it down:

style is loading the style-loader

!css is loading the style-loader. The ! is how loaders can be chained together

?modules&importLoaders=1&localIdentName=[name]__[local]___[hash:base64:5]&camelCase

There is a lot to digest here. Let's break it down piece by piece.

modules turns on css modules

importLoaders=1 The query parameter importLoaders allow to configure which loaders should be applied to @imported resources.

localIdentName is the localization format of the CSS class. We are using the format [name]__[local]___[hash:base64:5] which is the FileName__LocalClassName__12345

&camelCase sets the option to convert dash cased class names like my-class-name to be myClassName in your javascript

Install Your PostCSS dependancies

I recommend using:

npm i autoprefixer postcss-initial postcss-import postcss-mixins postcss-nested postcss-simple-vars postcss-math postcss-color-function --save-dev

Now setup a postCSS config file:

Inside /config folder create a postcss.config.js file

var postCSSConfig = [
/* autoprefix for different browser vendors */
require('autoprefixer'),
/* reset inherited rules */
require('postcss-initial')({
  reset: 'inherited' // reset only inherited rules
}),
/* enable css @imports like Sass/Less */
require('postcss-import'),
/* enable mixins like Sass/Less */
require('postcss-mixins')({
  mixins: require('../src/mixins')
}),
/* enable nested css selectors like Sass/Less */
require('postcss-nested'),
/* require global variables */
require('postcss-simple-vars')({
  variables: function variables() {
    return require('../src/variables')
  },
  unknown: function unknown(node, name, result) {
    node.warn(result, 'Unknown variable ' + name)
  }
}),
/* PostCSS plugin for making calculations with math.js  */
require('postcss-math'),
/* transform W3C CSS color function to more compatible CSS. */
require('postcss-color-function')
]

// Export the PostCSS Config for usage in webpack
module.exports = postCSSConfig;

Also create variables.js and mixins.js in your config folder

// src/mixins.js
var globalMixins = {
  /* noSelect is a static mixin  */
  noSelect: {
    '-webkit-touch-callout': 'none',
    '-webkit-user-select': 'none',
    '-khtml-user-select': 'none',
    '-moz-user-select': 'none',
    '-ms-user-select': 'none',
    'user-select': 'none'
  },
  /* OpenSans is a dynamic mixin  */
  OpenSans: function (obj, value) {
    return {
      'font-family': 'Open Sans, sans-serif',
      'font-style': 'normal',
      'font-size': value,
      'font-weight': 200,
      '-webkit-font-smoothing': 'antialiased',
      '-moz-osx-font-smoothing': 'grayscale'
    }
  }
}
module.exports = globalMixins

// src/variables.js
var globalVariable = {
  primary: 'blue'
}
module.exports = globalVariable

Now import the PostCSS config file into your webpack.config

// in /config/webpack.config.dev.js
/* import postcss config array */
var postCSSConfig = require('./postcss.config')

// in the webpack config add the postCSS Config Array.
module.exports = {
  entry: [...],
  output: {...},
  resolve: { extensions: [...] },
  resolveLoader: {...},
  module: { loaders: [...] },
  // Add postcss to webpack config object
  postcss: function() {
    return postCSSConfig;
  },
  plugins: [...]
}

Usage:

Using ClassNames

import React, { Component } from 'react';
import logo from './logo.svg';
import styles from './App.css';

class App extends Component {
  render() {
    return (
      <div className={styles.app}>
        <div className="App-header">
          <img src={logo} className="App-logo" alt="logo" />
          <h2>Welcome to React</h2>
        </div>
        <p className="App-intro">
          To get started, edit <code>src/App.js</code> and save to reload.
        </p>
      </div>
    );
  }
}

Variables + Mixins

You can now use the global mixins and variables!

Now in /src/App.css

/* in /src/App.css */
.intro {
  color: $primary; /* variable usage */
  @mixin noSelect; /* mixin usage */
  @mixin OpenSans 30px; /* mixin with value usage */
}

Profit