Assuming you already have the interface address for the guest vlan on the controller & associated with the SSID for the guest network, you could do two things.
1) Under the interface set the DHCP server to your current DHCP server and create a new scope on it. I would probably do this, because it makes managing DHCP for you network a little cleaner and, because the controller does DHCP relay, the hosts on the guest network will never see the IP address of your internal network. They will see the virtual address you configured on the controller as their DHCP server.
2) Log into the Controller and click on controller main heading. On the left you will see internal DHCP server - click on that. Click new and name the scope. The scope will then appear on the right, disabled by default. Click on the name of the scope and set all the scope options. Set the status to enable and you should be set. Like other Cisco devices you do not need to apply the scope to any SSID or interface. The device knows based on the network of the scope, what interface it should be using to hand out addresses.
