This extension integrates with the WordPress Customizer making it extremely simple to use. Once you enter the Customizer you will notice a new top-level section: Checkout.
The Storefront Checkout Customizer settings menu item will only display if you are viewing the WooCommerce Checkout page from within the customizer and the settings should be self-explanatory, but lets work through what each one does:
Here you can specify a preset layout option for your checkout.
The options are:
Default – the default layout
Stacked – Stacks the address inputs and order review atop one another
Two column addresses – arrange the address inputs into two columns above the order review
When combined with the full-width template already available in Storefront core this allows you to create 6 different checkout designs.