I cna't link to it or it'll get removed but if you google 'online Youtube MP3 ripper' you will find loads. Download your youtube files as specified in each ones instructions, then simply burn the MP3's as you normally would on your CD drive.
That adds a nice big 'Download As' button to your Youtube pages:
https://addons.cdn.mozilla.net/user-media/previews/full/188/188577.png?modified=1530209073
Clicking on it brings up various options, with 'MP4 720p' (if available) usually being fine for most purposes. However (if the video was actually posted in HD) there will also be a 'Full-HD' option shown. That, like the MP3 options, only works if Firefox thinks that you've made a small donation towards the development of the browser. If you'd rather not do so though, simply clicking on this link will give Firefox the cookie that it looks for:
https://videodroid.org/thanks_for_support.html
Thanks to you all, there seems plenty to choose from but I've opted for the Firefox extension recommended by Chris; and I already have the excellent Ashampoo programme.