Start with a known device type, e.g. KVM.
There is a video of the full process here:
https://www.linaro.org/projects/test-validation/
You've done some of the steps but things will be a lot clearer if you
follow the rest of the steps
This video may also help:
https://www.youtube.com/watch?v=M5kF3yPjYTk
The tools you use will depend on your particular lab. ser2net is common
and flexible for serial connections, many labs would find it much
better than using sg or screen. tftp transmission typically does not
need to be configured - the tftpd-hpa package is installed by
lava-dispatcher dependencies and can be used as-installed.