When you use Live Formatting, EasyRecipe looks for fonts on the page loaded from fonts.googleapis.com. It will automatically add these to the Live Formatting font selection drop down, so you shouldn't need to do anything to use Google fonts that your theme already loads.

If you want to use a font that isn't already loaded on the page and your theme doesn't have an option to add fonts, the easiest way to add a font is to edit your theme's style.css file and add the font @import at the end.

First, find the font you want to use at https://www.google.com/fonts

Then click on the Quick-use icon:

Scroll down to "3. Add this code to your website:" and click on the @import tab, then copy the @import url(https://fonts.googleapis.com/css?family=.... line and paste it into your theme's style.css file.