Woocommerce Checkout page customization

Reference:

  1. Official Tutorials
    https://docs.woocommerce.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/
  2. Sorting for checkout fields
    https://wordpress.stackexchange.com/questions/78339/how-to-reorder-billing-fields-in-woocommerce-checkout-template
  3. Woocommerce Hooks
    https://docs.woocommerce.com/wc-apidocs/hook-docs.html
  4. Other tutorial:
    http://uploadwp.com/customizing-the-woocommerce-checkout-page/