I'm doing this in my author instance. so I have a page (/content/my-site/my-page) with sling:resourceType = "core/wcm/components/page/v3/page". In /apps/core/wcm/components/page/v3/page, I created "page.print.html" with the following contents, <!DOCTYPE HTML>
<html>
<head></head>
<body>
...