Thanks to my professional experience with The Filthy Spectacula, I can now confirm the answer to this:
Feed people red wine at noon on Sunday. Gather a band, their wives and girlfriends, and the sound engineer round one vocal mike. Do not give them the chance to try and learn the ‘tune’ they’re going to ‘sing’ along to. Then triple-track the results.
All this, plus string quartets (three-quarters of them played by me … ), rock organ, handclaps, some genuine harmony vocals and all the Filthy hyperactivity you would rightly expect, now taped for the upcoming album Thr’p’ny Upright (*spelling tbc).