to add this custom checkout page , add the two pages of "form-checkout" and form billing in the folder of templates/checkout in woocommerce , also add custom css with plugin js and css custom , .. last thing the header.html insert it just in the page of checkout by plugin insert header and footer html code ... finally the custom design will be working , note : i used this custom code with theme avade : there is sample : https://sevenfiguresolo.com/checkout/
... if you want more thing , please never hesitate to open issue request and put question