Within the app settings, you will find the option for Automation.
Toggle to enable.
Once in the Automation menu, toggling the switch will bring up the Manage Automation option below. Tapping Manage Automation will display a screen requesting you to grant PIA access to location services. This is a necessary step. The purpose of location services being allowed is so that should there ever be two similar named SSID connections, your phone can differentiate between the two networks. You will need to turn on your device's location when you want to use the Trusted Networks feature.
You can then need to allow the app access to the devices location in order for it to discover the local Wi-Fi and Mobile networks in your area. Permissions can always be removed.
Once allowed, you will see the options for how you wish the VPN to react when connected to Wi-Fi or Mobile data in general.
Tapping on either option will display the possible rules you can set for the given connection.
Tapping the + Add new rule option beneath the available networks will then open a new window with the available Wi-Fi networks in your area.
Once added, tapping on a network will again bring up the possible rules you can set.
The network will then be added to Trusted Networks and can always be removed by tapping and selecting Remove rule.