Googling around, I found this to be buried in random forum posts. I figured I’d document this for future reference.
To transfer your Wix domain from Wix:
1. From your Domain Manager, under My Domains, next to your Wix domain, click Manage.
2. Under Domain Summary, click the Advanced tab.
3. Next to Transfer Away from Wix, click Transfer.
4. In the Transfer window, click Send.
5. An email with a code will be sent to you. Forward this code to your new domain registrar.
You may not receive the email immediately, but you can expect to receive it on the same day.
Source: Dana from WIX Support, Screen Caps: myself.
At this point, you will have to go to a new registrar, such as Namecheap.com and create an account. On the registrar side, you can initiate the domain transfer. Note: Be sure to jot down all DNS Zone information in the WIX Domain Manager panel. Be sure to update the new registrar with DNS information.