If you are talking about a 12V lead-acid battery, it is no different from any other wet car battery. Keep it upright so the acid doesn't spill. Cover the posts with something insulating so you don't short the terminals. Take it to a place that reclaims the lead in it, such as an auto parts store.
If you are talking about a traction battery, that's another story. The old one is worth a lot of money, even if old. Start with Nissan for recycling. They buy back Leaf battery packs.