Things You'll Need
Instructions
 
Travel to the Pokémon Center on Cinnabar Island. Use a Pokémon's "Fly" ability to get there quickly. 
Speak with Bill in the Pokémon Center, who will guide you to the new area of the game exclusive to "Pokémon LeafGreen" and "Pokémon FireRed:" the Sevii Islands. Use the island boathouses to travel to the fourth island, Floe Island. 
Walk to the building north of the Pokémon Center, the Day-Care. 
Speak with the attendant inside and ask to breed a male and female Snorlax. A Munchlax will hatch from the resulting egg.