To set up a guest network on the ZyXEL VMG8924 you will need to access the router interface through a web browser. To do this make sure you're on a device that is connected to the router, open a web browser, and then type the IP address of the router (192.168.1.1 by default) into the address bar then press enter. You will then be asked to log into the router.
By default the username is admin and the password is 1234, but the password should have been changed during the setup of the router.
Once logged into the router hover over the "Network Setting" option on the bottom menu and the select the "Wireless" option from the popup menu.
Once in the wireless section of the router you will see the "Guest/More AP" option of the top menu bar. Select this and then click on the modify icon next to the network you wish to modify.
Once on the edit page select "Enable" and then fill in the boxes with the setting of the network you wish to have. Selecting "Guest WLAN" will allow you to modify the access devices on this network, for example you can only allow the devices connected to this network to access the internet and not any local devices, like a NAS drive. Selecting different Access Scenarios will show pictures that give you a guide to what they do.
At the bottom of the page there is also the "Security Level" section. Changing the security level will change the wireless security mode (none/WEP/WPA2), it is advisable that you leave it on "More Secure" if all of your wireless devices support WPA2.
To change the wireless password un-tick the "Generate password automatically" box and then enter the password you want into the "Password" box. You can tick the "password unmask" box if you wish the characters to be visible.
Once the changes have been made click on "OK" at the bottom of the page.