Hi Megver83,
first of all, I am sorry that I am not going much into detail now, because I
don't have a lot of time.
I will come back to this topic. ;-)
You don't need a PSU! The 3.3V of the RPi is sufficient. The Jumper cables
are only different in colour so that you can identify connections more
easily. It makes it more comfortable to work with.
A set of jumper cables is very cheap, so I would use (and have used) jumper
cables bought from a shop because they have a header that makes sure that
everything stays where it should. You don't want to lose a cable during
flashing.
Make sure that you have everything you will need: Pomona Clip, Jumper Cables,
RaspberryPi ready to go (I forgot what else there is).
There is another manual for flashing with a Raspberry Pi. There is not much
different when flashing a T400. Just be careful with the pinouts.
https://libreboot.org/docs/install/rpi_setup.html
The manual is very detailed, just make sure to follow it closely:
https://libreboot.org/docs/install/t400_external.html
Good luck and do not hesitate to ask for help.
