To set catalog checkout page please go to Product Settings > Order Form (on the left menu in General Settings tab).
The first section of Order Form Settings is Order Form Pages. Use the Order Form Page drop-down to select the page which will show the order form.
Order form shortcode
After the eCommerce Product Catalog checkout page is selected please go to the selected page edit screen. In order to finish the order form checkount page configuration you should paste [order-form] shortcode on the selected page.
That’s it. Now the selected page will work as a checkount page for your eCommerce Product Catalog. When the page is wisited directly it will automatically redirect the user to the products page with the information that the product should be selected first.
When the order button is pressed on the product page than the user will be redirected to the checkout page. When the summary and the order form will appear.