Instructions
Collect at least 100p (in-game currency) and head to Floaroma Meadow in Floaroma Town and find the honey for sale. Buy at least one jar of honey at 100p apiece.
Find a honey tree. The honey trees are distinguished by having gold leaves. There are 21 honey trees spread throughout the game world. There is one in the Valley Windworks, one in the Floaroma Meadow, one near the Eterna Forest, one near the Fuego Ironworks and several scattered along various routes.
Approach the honey tree and press "A" followed by "Yes" to slather the bark with honey. You must now wait eight in-game hours (which are eight minutes in real time).
Go back to the honey tree after approximately eight in-game hours and it should be shaking. Approach it to initiate a battle with a Pokemon. Repeat the process of buying honey and slathering it on the tree until you are paired up with a Munchlax.
Battle and capture the Munchlax in the same way you would any other Pokemon. Hammer it with attacks until it is weak and then capture it with a Pokeball to add it to your menagerie.
Fight various other trainers and Pokemon using your newfound Munchlax until it evolves into a Snorlax. This may take several hours of game play depending on how well your Munchlax fights. Munchlax will evolve when his happiness rating reaches over 220.