(this page mostly done, possible missing steps that can be found on https://github.com/timvideos/litex-buildenv/wiki/Using )

1. sudo apt install build-essential
2. litex-buildenv (click and do, before going to the next step)
3. get / setup the HDMI2USB-litex-firmware

git clone https://github.com/timvideos/HDMI2USB-litex-firmware.git
cd HDMI2USB-litex-firmware
export PLATFORM=opsis    # Or atlys
scripts/download-env.sh

export PLATFORM=opsis
export TARGET=hdmi2usb
source scripts/enter-env.sh
make gateware
make image

The built gateware will be in build/opsis_hdmi2usb_lm32/gateware/

To flash to a plugged in device:

make flash

May also be necessary to set the CPU:

export CPU=lm32

which used to be the default CPU, but the default CPU is now vexriscv which hasn't been tested much with HDMI2USB. If doing that, set CPU=lm32 both when running scripts/download-env.sh and when building/installing (otherwise the wrong tools will be installed, or the wrong version will be installed).