How does the spell double double toil and trouble go?

Bubble, bubble, toil and trouble, is a quote from Shakespeare. The witches that Macbeth visits chant this, around a steaming cauldron. As to the actual spell, it seems to be related to their ability to predict the future. Whether modern witchcraft uses this, I have no idea. From the "spells" that some modern witches use, it seems likely. To quote Cecil "El retardo supremo, if you want an opinion."