How do you clone Pokemon on Pokemon HeartGold?

Answer 1

You can not unless you use third party modifiers like an Action Replay.
you can get it if you have a pokewalker and translate a pokemon into it and when its saving you turn the game off and there you have two pokemon

Answer 2

You must have an action replay, and you must have the cloning cheat. To find the cloning cheat, go to Supercheats.com