Basically what the title says: how would you go about integrating this player into an already existing HTML? 

Hi, you have a couple options:

1. upload the player files to some location on your server, then add an iframe on your page that points to the index.html that you uploaded.

2. upload the supporting files into the directory your html page exists (or at another location, but you’ll have to update references), and add in the player’s styles, html, and scripts into your page (or vice versa, adding your page’s content into the scritch player’s page)