Instructions
Catch Sudowoodo. If you do not already have Sudowoodo, you can find him at the southernmost point of Route 37, which is north of the Ruins of Alpha. You will not be able to fight him unless you first acquire the squirt bottle from Goldenrod City and use the item on him.
Catch a Ditto. Ditto is significantly less rare than Sudowoodo and can be found throughout Routes 34 35 and 47, as well as the Safari Zone and Cerulean Cave, although Route 47 provides the best opportunity to find one.
Go to Route 34, which is south of Golden Rod City. Go inside the Pokémon Daycare Center building, indicated by an older man standing beside the door.
Place both the Ditto and Sudowoodo in the daycare. They will eventually make an egg, which then hatches into Bonsly.