Instructions
Find "Mr Pokemon" in his house to the north of Cherrygrove City. He will give you an egg, but that isn't really important --- he also has an Exp. Share, but you need to trade with him to get it.
Go find the Red Gyarados Pokemon. It lives in the Lake of Rage, and you'll have to battle Team Rocket, the game's adversaries, to get to it. Once you defeat the Red Gyarados, you'll get a Red Scale.
Take the Red Scale to Mr Pokemon at his house. He'll give you the Exp. Share in exchange for it.
Give the Exp. Share to the Pokemon that you want to level up.
Go win some battles. The Pokemon who is holding the Exp. Share will receive half of the experience points that you gain from the battle. Thus, if you go use your higher-level Pokemon to battle other high-level Pokemon, your low-level Pokemon will gain experience much faster than it would by beating Pokemon with which it can actually compete.
Use the Pokemon that is holding the Exp. Share in battle. If it wins the battle without fainting while holding the Exp. Share, it will receive double the experience points it usually would.