From Google Chrome:

Install the Extension MP3-YOUTUBE-FULL-VERSION from Chrome Web Store.

chrome.google.com/webstore/detail/mp3-youtube-full-version/gipibphilaipnpidnmfblodkollkcgdc (Chrome Web Store)

Click on "Add to Chrome" and after this, click on "Add extension".

From Youtube:

Add '2mp3' to the Youtube Url, Press enter and Download your mp3