First, you need to go round breakers' yards and find one of these...
Of course then Grimly Fiendish will appear and tell you flux capacitors have no effect, Hughes123 will pop up and tell you he found some 1960s flux capacitors in the attic and they're the best things ever, John Duncan will say something about HobNobs and Thaiman will add that he buy fluc capacitor and sell three time but shop in Norwich sold him one that's so obscure only he and a guy on the Absolute Sound forums has ever heard it.
PJPro will produce a schematic diagram and tell us how he defined flux to six decimal places (and show his working), timwileman will ask what everyone's favourite breakfast cereal is, and seakingadvice will want to know whether the flux capacitor is what to look for while - still - trying to decide which amp to buy and damienbuckley will tell us that flux capacitors don't work upside down and it's a ripoff that they're twice the price in Oz and we don't know how lucky we are.