What do you need to know? The firmwares all come with their own documnentation but basically you have to tell it which pins are used for the rows and columns and which key is where (this is deliberately vague as you've not said which firmware you want to use...)
I'm an EasyAVR fan so I would load the 104 key layout from http://keyboard-layout-editor.com into the 'define keyboard' wizzard using the pins I want to use for rows and columns and it will guess the matrix - look at the bottom of the .py file it makes in c:\users\USERNAME\.EasyAVR\boards (it's just text so you can open it in notepad) and you'll see the keys listed in order, the second set of braces are (row, column) so the easiest way would be to match your wiring to that.
I have never attempted to configure a custom board in TMK/QMK as it's all a bit strange, but people do so if you need help with that say so
Once again, this makes so much more sense than any tutorial I found googling this!
I really don't know firmware or coding, so I dunno what I want to use really. I'm a mechanical engineer, very much biased towards the mechanical side. I know forces, calculations and constructions, but software and coding are very much alien to me.
The way you make it seem is awesome tough! Basically I just copy the wiring it makes for me, and then load that piece of firmware onto the teensy?
Thanks for sticking by my noobishness