go and visit that nice google fonts helper site: Link
Look for the font you like to implement in your wordpress. Don’t forget to mark all the font-weights you like to have.
Copy/Paste the css codes into a file – name it as : font.css
so f.e. open-sans.css, oxygen.css etc. pp.
Download the zip file and open it.
Now prepare your child theme folder: Create a folder called : assets
in this folder put a css and a fonts folder
put all the fonts downloaded into fonts-folder
put all the font.css (open-sans.css, oxygen.css etc.pp) into the css-folder