Hiya, clone the github here:
https://github.com/zhoulander/Powermon433MQTT
Compile the .INO file and upload it to your UNO, plug it into your Pi, and edit and run the Python script to reflect your own MQTT server address.
The steps in the readme should be straight forward as long as you have a little bit of Linux experience (file editing, installing Python, finding your Arduino COM port)
If you have Home Assistant and are using the add-on store MQTT Broker make sure you follow the documentation on how to allow anonymous logins and how to setup unrestricted access to all topics via the ACL files.