Instructions
Go to Goldenrod City in the south west of Johto and defeat the Pokemon Gym Leader, Whitney. She has a Level 17 Clefairy and a Level 19 Miltank, both of which are very weak against Fighting-type Pokemon.
Go to the pink house just to the west of the Goldenrod Gym. Speak to the lady inside and she will give you the Squirtbottle.
Head north on Route 35 and then east onto Route 36. Here you will find the path blocked by a dancing tree, which on closer inspection turns out to be a Sudowoodo.
Select the Squirtbottle from the Key Items pocket of your bag and use it on the Sudowoodo. It will spring to life and attack you.
Wear down Sudowoodo's health points with Water, Grass, Fighting, Steel or Ground-type attacks. Once its health is very low and is showing in red, throw a Pokeball to catch it.