Instructions
Place any of the Mudkip evolutions in your Party. The Mudkip evolutions include Mudkip, Marshtomp and Swampert.
Place a generic breeder in your Party as well. The generic breeders are Ditto and Kecleon.
Give the Mudkip evolution and the breeder Pokemon to the Day Care man, located inside the Pokemon Day Care.
Talk to the Day Care man every 500 steps and he will eventually give you a Pokemon egg. The egg will hatch into a Mudkip.