Instructions
Obtain an Odd Incense. In "Pokemon Diamond," "Pokemon Pearl" and "Pokemon Platinum," you can find an Odd Incense in the Solaceon Ruins. In "Pokemon HeartGold" and "Pokemon SoulSilver," look for an Odd Incense in Cerulean Cave. In "Pokemon Black" and "Pokemon White," you can buy an Odd Incense for $9,600 from the Driftveil Market.
Catch one male Mr. Mime and one female Mr. Mime. Alternatively, catch one Mr. Mime of either gender and one genderless Ditto.
Give one of the Mr. Mimes, either female or male, the Odd Incense to hold. Take the Mr. Mime pair -- or one Mr. Mime and a Ditto -- to a Pokemon Daycare.
Talk to the daycare owner repeatedly, until you receive an egg from the breeding pair. This may take up to one day.
Walk around with the Mr. Mime egg in your party. After your character takes about 6,600 steps, the egg hatches.