How do I setup a guest network with Time Capsule
The in app help will guide you if you search for guest or "set up a guest network"

The settings are under wireless with a simple check mark to enable a guest network.

Here is the most relevant steps from the help included with Airport Utility:

To set up a guest network, the Apple wireless device must be set up to share its public IP address using DHCP and NAT.

1. Open AirPort Utility, located in the Utilities folder in the Applications folder.
2. Select the wireless device on which you want to set up a guest network, and then click Edit.
3. Click Wireless, and then select Enable Guest Network.
4. Give the guest network a name.
5. Choose None, WPA/WPA2 Personal, or WPA 2 Personal from the Guest Network Security pop-up menu, and then type a password for the guest network.