To create a guest WIFI network login to your router using your routers IP address.
Once logged into your router look at your settings and look for an option called guest access or something similar it will be different depending on the make of your router so you'll have to search around for the settings.
In the guest access menu select allow guest access or allow guests whichever your modem or WIFI router calls it.
You should be able to setup a guest password or leave it open without a password.