Using 301 redirect to change my domain

Hi,

Looking for clarification on how to use a 301 URL redirect. I changed my primary domain about a year ago (incidentalsewing.com -> incidentalwildland.com) but still currently own both domains. I understand that in order to use a 301, the old domain needs to be disabled or deleted. I think I'm ready to do that, but before I do I have two questions:

1. How can I see how much traffic if any is still using the old web address, and can I see where they're coming from?

2. How do I disable the old domain? Do I just let it expire?

Thanks!