Instructions
Go to the Pokemon Daycare Center. This is on Route 3 in Pokemon "Black" and "White" versions, to the west of Solaceon Town in "Diamond", "Pearl" and "Platinum", on Route 34 in "HeartGold" and "SoulSilver", and west of Mauville City in "Ruby" and "Sapphire" editions.
Put a male Pokemon and a female Pokemon into the Daycare together.
Leave and and talk to the old man outside the daycare. If he says "The two seem to get along," there is a 50 percent chance of them creating an egg. If he says "The two seem to get along very well!" there is a 75 percent chance. However, if he says that "The two don't seem to like each other," then there is only a 20 percent chance. If the man says "The two prefer to play with other Pokemon more than with each other," then there is no chance of them mating, so go back in and try a different pairing.
Go for a walk. Every time you go 256 steps, your Pokemon have a chance of mating. For instance, if the old man talked about them getting on "very well," then there is a 75 percent chance every 256 steps.
Visit the Daycare Center after walking at least 256 steps. If the two have mated, the old man will call you over and give you an egg.