Instructions
Capture a Ditto from the trophy garden behind Mr. Backlot's house on Route 212, south of Hearthome City.
Capture a Yanma in section six of the Great Marsh Safari Zone, located in Pastoria City.
Travel to Solaceon City, located south of Route 210 and west of Hearthome City.
Enter the daycare center on the west side of town. There is a sign in front of it that displays "Pokemon Day Care" when it is read.
Speak to the woman behind the desk. She asks for two Pokemon to watch over.
Select both Ditto and Yanma from the list of Pokemon.
Exit the daycare center and take 256 steps outside. After taking 256 steps, speak to the elderly man in front of the daycare center. If he does not have a Pokemon egg for you, keep taking 256 steps until he gives you an egg. This egg contains Yanma, as Ditto cannot produce eggs of its own so the resulting egg is always of the other Pokemon of the pair.
Walk 5,355 steps while the egg is in your inventory. After taking the appropriate amount of steps the egg hatches into level 1 Yanma\.