It’s enough to do it like this tutorial says:
https://wiki.openwrt.org/doc/recipes/guest-wlan-webinterface

If you want a log of the surfing history for maybe avoid being held responsibly for the things your guest does (Störerhaftung) you can log the DNS queries:
https://superuser.com/questions/632898/how-to-log-all-dns-requests-made-through-openwrt-router/897413#897413

Now you need a rsyslog server that collects those logs:
https://kimkil.nietsniehu.net/raspberry-pi-as-a-log-server-with-rsyslog-2/