gulpfile.js

'use strict';

var gulp = require('gulp');
var sass = require('gulp-sass');
var concat = require('gulp-concat');
var minifyCss = require('gulp-minify-css');

gulp.task('sass', function () {
  gulp.src('./source/sass/**/*.scss')
    .pipe(sass().on('error', sass.logError))
    .pipe(gulp.dest('./assets/css'))
    
    .pipe(concat('app.min.css'))
    .pipe(minifyCss())
    .pipe(gulp.dest('./assets/css/'));
});
 

gulp.task('sass:watch', function () {
  gulp.watch('./source/sass/**/*.scss', ['sass']);
});

setup.md

Console:

npm install --save-dev gulp
npm install --save-dev gulp-sass
npm install --save-dev gulp-minify-css