Clone the repo and get dependencies via composer

git clone --depth 1 --branch master https://github.com/phpish/shopify_app-skeleton.git new_prj
cd new_prj
rm -rf .git
rm README.md
curl -sS https://getcomposer.org/installer | php
php composer.phar install

Create your app. Use http://path-to-new_prj/oauth.php as the Callback URL

Get the app's credentials and update conf.php

Intall the app on a store by visiting http://path-to-new_prj/install.php?shop=example-shop.myshopify.com

An OAuth token for the shop should now be stored in the session. Check out the other .php files and see how they work.