gulpfile.js

var gulp = require('gulp');
var sass = require('gulp-sass');

gulp.task('sass', function () {
    return gulp.src('style.scss')
        .pipe(sass().on('error', sass.logError))
        .pipe(gulp.dest(''));
});


gulp.task('watch', function () {
    gulp.watch('style.scss', ['sass']);
});

install.sh

// install npm from: https://docs.npmjs.com/getting-started/installing-node 
$ npm install --global gulp-cli
$ npm install --global gulp-sass
$ cd path/to/project
$ touch gulpfile.js
$ gulp watch

package.json

{
  "repository": {
    "private": true
  }
}