Login to your BigCommerce back office and navigate to Advanced Settings -> Checkout.
Change the "Checkout Type" to be "Custom Checkout".
Scroll down to the "Custom Checkout Settings" and set the "Script URL" to be https://cdn.ebizio.com/ebizio-checkout/storefront/auto-loader.js.
Check the box stating that "This checkout supports Optimized One-Page Checkout settings". This is required for the checkout to work properly.
Check the box stating that "Use the same script URL for the custom order confirmation page.". This is required for the order confirmation page to work properly.
Click "Save" to the bottom right
If you do not have the option to enable a Custom Checkout, you will first need to have the "Optimized One-Page Checkout" active on your store. Typically, once you enable this option you will see "Custom Checkout" as a choice.
note: if "Optimized One-Page Checkout" isn't already active, please read item #2 here as there are some considerations to make prior to switching.
If "Optimized One-Page Checkout" is currently active and you still do not see "Custom Checkout" as a choice, please submit a support ticket we can put in a request with BigCommerce.