I download youtube videos in .mp3 format very often, both for music and to listen to as podcasts. I don't know if it's the best method but this is how i do it:
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Use the MultiSelect extension to select multiple videos and put them in a playlist, which i name Download. This playlist has to be public.
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Use Open Video Downloader to download this playlist, making sure that it writes the metadata in tags. If necessary, it can also download age-restricted videos by importing cookies from your browser, i've done it a couple of times but it's not convenient.
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I pretty much leave it at that, but you could use a media tagger to add further information to the files.