You have to get next to him and go to your pokegear phone ang go to the very top

You can't get snorlax in kanto, although you can catch him when you wake him up in johto.
If you killed snorlax accidentally or ran away, re-battle the elite four and snorlax
should appear at the same spot for you to catch.

If i remember right, after you get the radio in lavender town, you go to the snorlax and
use the pokéflute (go on the radio and put the thing right at the top. ) this should wake
snorlax right up!