https://openwrt.org/docs/guide-user/network/wifi/guestwifi/configuration_webinterface
I was able to follow these instructions (even though some of the screenshots were outdated) and I have a guest wifi working.
But why did I have to do so much work in the first place? Why can't OpenWRT have a simple "Enable Guest WiFi" option that does all this work one one step? The only thing I might want to configure is the range of IP addresses on the 192.168.2.x network and maybe the lease time. But everything else should have been set up by default.