Postcss startup files

gulpfile.js

var gulp = require('gulp');
var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer');
var precss = require('precss');
var cssnano = require('cssnano');
var watch = require('gulp-watch');

gulp.task('css', function () {
  var processors = [
    autoprefixer,
    precss,
    cssnano
  ];
  return gulp.src('./src/*.css')
    .pipe(postcss(processors))
    .pipe(gulp.dest('./dest'));
});

gulp.task('watch', function(){
    watch(['./src/*.css'], function(event, cb) {
        gulp.start('css');
    });
});

gulp.task('default', ['watch']);

package.json

{
  "name": "postcss-test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "devDependencies": {
    "autoprefixer": "^6.0.3",
    "cssnano": "^3.3.1",
    "gulp": "^3.9.0",
    "gulp-postcss": "^6.0.1",
    "gulp-watch": "^4.3.5",
    "precss": "^1.3.0"
  }
}

Install all npm modules by npm install, run gulp.