https://libreboot.org/docs/install/t400_external.html
https://libreboot.org/docs/install/bbb_setup.html
You should make a list of things you will need, for example a new Wifi card
(e.g. Atheros), a Pomona SOIC clip, a Raspberry Pi or a Beagle Bone Black
(there are other devices) +display, thermal paste, pin header/ jumper cables
and so on.
You also need lots of space to work and a little time. You can and should
take photos in the process of disassembling and reassembling the T400 in
order to help you remembering each step. You can draw the board and other
parts of the T400 on paper and put the screws where they belong. I did this
to help me with every step. It will be easier in the end.
I used a Raspberry Pi because I had one already. You will need to know about
the pinout of your flashing device, in my case the Pi:
https://pinout.xyz/
If you're not all thumbs it isn't that hard. It is rather time consuming and
a little annoying. ;-)
