You'll use either a schematic of the machine or your engineering background with knowledge that ground is all over. If you can't find ground I think you may be in over your head.
Sorry I don't want to upset you. Once in a while you find folk that think there is a step by step to turn a LCD into a display. Well there is. Repair the laptop and it works again.
And again, see the web for examples of how many got this to work. It's very few and most had electronic/electrical degrees.