For example, we want use Open Sans locally. On Google Fonts, in "Open Sans", click "Add to Collection". and click "use".
Under "Choose the styles you want:", check all items, under "Choose the character sets you want:", check "Latin" and "Latin Extended".
Open the url in the 3rd step, here it is:
https://fonts.googleapis.com/css?family=Open+Sans:400,300,300italic,400italic,600,600italic,700,700italic,800,800italic.
Now you get a css file, save it into $WEBSITE_HOME/client/stylesheets/font/opensans.less, delete all sections other than "latin" and "latin-ext", download