This article only covers the Shopify specific setup, for the full setup use  Setting up a new Webstore

Configure Custom App in Shopify

1. Log in to your Shopify admin portal / backend.
   This is done by going to  https://www.shopify.com/login  and entering your username and password.
2. Click on Settings  on the left. 
   
3. In the top left corner, note down your Shopify URL, for example: yourstorename.myshopify.com
4. Cick on Apps and Sales Channels on the left.
5. Click on Develop  Apps  on the top right. Then, click Allow Custom App Development if it appears. Confirm, if it asks.
6. Click on  Create an App .
• In  Title , type in a name for the private app eg.  Retailer Link .
  This is only for identification purposes.
• Click Create App .
• Click on Configure Admin API Scopes .
  
• Under Admin API access scopes , tick all the boxes that say read or write. These give the link permissions to update the website.
  
• Under Webhook Subscriptions , make sure the Latest version is selected. Click Save at the bottom.
  
• Click on API Credentials near the top.
  
• Click Install App under Access tokens. Confirm by clicking Install .
  
• In the new line that appears, click Reveal Token once and copy or note down the revealed code. You can only do this once so make sure you have it recorded before continuing.
  

Setup Customer Checkout options

1. On Shopify, go to Settings > Checkout
2. In Customer Contact, select  Customers can only check out using email.