Instructions
Catch wild Pikachus in the Safari Zone until you get one that's holding a Light Ball. Pikachu is a rare encounter, and it will only have an item around 50 percent of the time, so this may take a few tries. Having an Electric-type Pokemon in your party while hunting in the Safari Zone will make it more likely for a wild Pikachu to attack you. Leading with a Pokemon that has the ability Compoundeyes, such as Butterfree, will make it more likely that the Pokemon you encounter will be holding an item.
Place the Pikachu with the Light Ball along with a second Pikachu or a Raiku in your traveling party of six Pokemon.
Go to the Pokemon Daycare Center, just west of Mauville City on Route 117.
Put the Pikachu with the Light Ball and the second Pikachu or the Raiku in the Daycare together.
Go for a walk. Every time you walk 256 steps, there is a chance that the two Pokemon in the Daycare will mate and produce an egg.
Go back to the Pokemon Daycare and talk to the old man outside. If your Pokemon have mated, he will give you an egg.
Place the egg in your traveling party of six Pokemon, and keep walking around. Every 256 steps, there is a chance that the egg will hatch. When it does, it will yield a Pikachu that knows Volt Tackle.