Plug it into a light and let it go until it runs out. Your charger (if you have a decent smart charger) probably has a kill/destroy/depose setting. That purposely burns the battery out.
When I do it I use the charger first. Then once that charger cycles it out, then I'll plug it into a light. You can usually find some cheap battery killer lights like on GetFPV or Racedayquads. I plug that in and let it sit for a couple of days until the light is completely out to ensure it's dead.
Then find yourself a battery recycling place and drop it off.