npm install extract-text-webpack-plugin --save-dev

# for webpack 2
npm install --save-dev extract-text-webpack-plugin
# for webpack 1
npm install --save-dev extract-text-webpack-plugin@1.0.1

const ExtractTextPlugin = require("extract-text-webpack-plugin");

module.exports = {
 module: {
  rules: [
   {
    test: /\.css$/,
    use: ExtractTextPlugin.extract({
     fallback: "style-loader",
     use: "css-loader"
    })
   }
  ]
 },
 plugins: [
  new ExtractTextPlugin("styles.css"),
 ]
}

const ExtractTextPlugin = require('extract-text-webpack-plugin');

// Create multiple instances 
const extractCSS = new ExtractTextPlugin('stylesheets/[name]-one.css');
const extractLESS = new ExtractTextPlugin('stylesheets/[name]-two.css');

module.exports = {
 module: {
  rules: [
   {
    test: /\.css$/,
    use: extractCSS.extract([ 'css-loader', 'postcss-loader' ])
   },
   {
    test: /\.less$/i,
    use: extractLESS.extract([ 'css-loader', 'less-loader' ])
   },
  ]
 },
 plugins: [
  extractCSS,
  extractLESS
 ]
};