Instructions
Obtain the Full Incense item, which can be found in Mt. Mortar.
Head to Route 11 in Kanto to find Snorlax.
Battle Snorlax, lowering its HP into the red. Put Snorlax to sleep or paralyze it, if possible, then use a good Pokeball to capture it.
Make Snorlax hold the Full Incense. Without the Full Incense, you will breed another Snorlax rather than a Munchlax.
Take Snorlax to the Pokemon Day Care center on Route 34 and breed it with another Pokemon. If you have a male Snorlax, you will need to breed it with Ditto to obtain Munchlax. A female Snorlax can be bred with any Pokemon in the same Egg Group; so long as Snorlax holds the Full Incense, the resulting egg will hatch into Munchlax.