"It’s simple, really," Harry replied, darkly. "The process was always the same ... I’d screen the escapees at The Burrow, then again at Merlin’s Cave, just to make sure that they hadn’t been replaced ... or Polyjuiced ... in the intervening time. Once the Weasleys had acquired a family of four, Polyjuicing into them was the easy part ... hell, you could brew Polyjuice when you were barely a teenager! The recipe is readily available in a Hogwarts text book!