Do you want to measure just one appliance, running at 120 V?
If you want to do that with an Arduino Uno and feed the output by a wired connection into a Raspberry Pi, then you need to follow the basic instructions in ‘Learn’ - Interfacing with an Arduino to get the current measuring part working, then to connect that to the GPIO of your RPi, follow Direct connection emonTx V3 > Raspberry Pi GPIO in the Wiki via Resources → EmonTx V3 → Hardware Docs.
Why do you ask? Are you still struggling with your Shield?