As it comes up quite a bit, and as IPv6 is, well, just different than IPv4, trying to find some good documentation on what the current recommendations are can be difficult. IPv6 and practices around it have evolved immensely over the last ten years.
I just came across this IETF reference, presently dated March 19, 2018, which at least gives a very high-level summary and references to specific RFCs and sections.
While it requires a basic understanding of IPv6, I'm finding it a lot more useful than my hodge-podge of bookmarks of sites with outdated recommendations.
And if you're looking to "get your feet wet" with IPv6, a good place to start is Hurricane Electric's free Tunnel Broker and IPv6 Certification.
A Tunnel Broker account lets you create a "6in4" tunnel. This is useful if your ISP doesn't support IPv6. After the tunnel is installed, your router takes each IPv6 packet, wraps it in IPv4, sends it to the Hurricane Electric tunnel machine, which unwraps the IPv6 packet and sends it on its way. https://tunnelbroker.net/
The IPv6 Certification is a set of tasks that you perform after you have the 6in4 tunnel set up. They really help you get comfortable looking at IPv6 addresses, DNS AAAA records, and other stuff you need to know. https://ipv6.he.net/certification/
There's a page on the OpenWrt wiki that describes how to install the 6in4 tunnel software on the router. I don't have access to the URL right now.
Lots of US ISPs these days have their own ipv6, so that close to 40% of Google traffic is already ipv6 in the US. So check to see if you already have ipv6 before setting up your tunnel
The HE tunnelbroker.net site also has a tool that generates the needed UCI commands for OpenWRT. It also includes the opkg commands needed to install 6in4. Just click on the "Example Configurations" button after you select the tunnel from the list.
You should have an IP assigned to your WAN6 Interface if your ISP runs native IPv6.