Instructions
Explore Mt. Mortar, north of Route 42, to find the Full Incense on the ground in the second area of the first floor.
Open the item menu and select the Full Incense. Give the Full Incense to Snorlax as a hold item.
Visit the Pokemon Day Care Center on Route 34. Talk to the woman behind the counter to leave two Pokemon behind so that they can attempt to breed. Select Snorlax and another Pokemon of the opposite gender in the Monster Egg Group, such as Charizard, Mudkip and Cubone.
Walk around for awhile and come back to the Day Care Center to see if your Pokemon have laid an egg. Once the egg is ready, you must agree to hatch the egg.
Walk 10,240 steps to make the Munchlax egg hatch. The fastest way to walk multiple steps at once is to use your bike since this is still considered walking.