Ok, the MightyHat is assembled.
Do I need to upload a sketch for it to work?
What should be in the sketch?
How do I upload it?
Upload like to any arduino through FTDI/serial interface (while disconnected from the Pi is best).
Sample sketch for it to act as RFM69 IoT Gateway is here:
https://github.com/LowPowerLab/RFM69/blob/master/Examples/MightyHat/MightyHat.inoFrom that sketch you can gut what you don't need and have a great starting point for whatever else you want to do.