1. Obtain the Silver Badge: This is obtained after defeating the Gym Leader Whitney in Goldenrod City.
2. Talk to the Old Man in Goldenrod City: After obtaining the Silver Badge, head to the small house next to the Goldenrod Department Store. Inside, you'll find an old man who will give you the Pokeflute.
Once you have the Pokeflute, you can use it to summon Snorlax from his slumber at the following locations:
* Route 12: Near the entrance to the Safari Zone.
* Route 16: Between the Victory Road and the entrance to the Indigo Plateau.
* Route 36: Near the entrance to the Lake of Rage.
The Pokeflute is a useful tool for catching Snorlax, as it is a very powerful Pokemon that can be difficult to catch otherwise.