Detailed documentation you may find at https://developer.bigcommerce.com/stencil-docs/customizing-checkout/installing-custom-checkouts
In order to use custom checkout you need to create a fork from https://github.com/bigcommerce/checkout-js
In order to build and deploy you should install required dependencies at just downloaded repository. And build the checkout
npm ci
npm run build
NOTE: you may not be able to build the checkout from windows OS.
You will need to host the custom checkout file online so it can be served by the store. You can use a hosting service, such as Amazon S3. Using an external host will allow you the freedom to automate the build process if you wish and push updates automatically from your local machine.
You can upload a custom checkout to your store’s server using WebDAV. Just upload all files and folders from dist folder into webDAV content/checkout folder.
In order to enable custom checkout just navigate to Open Control Panel > Settings > Checkout.