This is really easy now. Put special Style Manager block on page and simply put whole html (you can copy even full html with head, footer, plugin will clean them up) and click to convert.
Plugin will convert html to blocks, it will add styles as classes, custom css to custom css section and all JS to custom JS section of style manager
Style manager has also many useful functions. For example, if you remove some blocks on page, you can select to clean unused classes.
Check video