How to use the latin-ext subset of Google Fonts font?

Can someone explain in detail, how to add support for Google Fonts latin-ext subset to my website (only for headers h1-h4)?

I’ve seen several articles on the web (including Google Fonts site) that deal with this topic, but not in the context of using RapidWeaver. Besides, they all presumed the advanced level of knowledge about using web fonts – for me, it is a new concept.

I have the Calligrapher stack from Doobox which supports Google Fonts, but so far I am not sure how to use it in that capacity.