Instructions
Capture a Ditto from the cave in Cerulean. A Ditto can breed with any Pokemon in the game. Otherwise, you would need to have two Pokemon of the same egg group to breed.
Go to the Day Care Center on Island 4. The prerequisites to visit Island 4 include:
Place the Ditto into the Day Care Center with another Pokemon.
Walk back and forth in front of the Day Care Center until the egg is ready. Egg hatching times may vary, but they require you to take a certain number of steps. Standing still outside of the Day Care will not allow the egg to hatch.
Talk to the old man from the Day Care Center when he steps outside of the fence.
Make sure you have only five Pokemon in your group so that you can receive your egg. The Pokemon you receive will be the first evolution of the kind of Pokemon you placed in the Day Care with the Ditto.
-Logging more than 60 Pokemon into the Pokedex
-Battling the E4 and becoming the Pokemon Champion
-Receiving the National Dex
-Retrieving the Ruby from Mt. Ember.