It's a very complicated process. First you need a Zigzagoon that knows Flail. Then you breed it with a Farfetch'd. Take the Egg and hatch it into a Farfetch'd which should then know Flail. Take the Farfecth'd who learned Flail and breed it with a Doduo. This should finally result in a Doduo egg which, when hatched, should know the move Flail. You can look this sorta stuff on Bulbapedia, but I'm glad I could help anyways.