Assuming you are running on a Raspberry Pi 2-4 or Zero (tested with bullseye):

1. Install all necessary packages to synthesize rot.v and build bit image (rot.bin) for the ice4pi:

 sudo apt-get install yosys fpga-icestorm arachne-pnr
 make

2. Make sure your Pi has SPI enabled (e.g. use raspi-config)

3. Building and installing the flashrom tool

 git clone https://github.com/lightside-instruments/flashrom.git
 cd flashrom
 make CONFIG_ENABLE_LIBPCI_PROGRAMMERS=no CONFIG_ENABLE_LIBUSB0_PROGRAMMERS=no CONFIG_ENABLE_LIBUSB1_PROGRAMMERS=no install
 cd ..

4. Load the rot.bin file to the shield:

 make load