If you already got the EXPN Upgrade for your Pokegear, you already have the Pokeflute. You get it by going to Ceruleun City. Then, you must find and defeat the Rocket Grunt and he'll tell you where the missing part from the machine in the Power Plant is. Search and get it at the Celuleun Gym using the Dowsing Machine which you get from a person in Eruteak City.
After you got the Missing Machine Part, go to the Power Plant and return the part to the owner or whoever he is. He gives you a TM which teaches your Pokemon Charge Beam.
Now, go to the Radio Tower in Lavender Town. Inside is a man who looks like a Gentlemen. Talk to him and he gives you an upgrade for your Pokegear so you can listen to the radio in Kanto.
Next, go to Route 11. (The place where the Snorlax is)
Go to the Radio Section (while you are on Route 11) in the Pokegear and move the cursor to the very top of the circle....then the Pokeflute music plays. Exit out of the Pokegear with the Pokeflute music still playing. Save, then go up to the Snorlax and the battle begins.
The Snorlax as at Lv.50, has moves like Roll Out & Block, and has a chance of holding a Leftovers item...a very good item indeed. Use whatever Pokeballs you like, though the Heavy Balls are recommended. Heavy Balls are made from Blk Apricorns...and you must give them to Kurt so he can make the Heavy Ball.