How to fix external mic input not being picked up on a computer

Ok, just a quick video on on how to enable an external microphone on your laptop. I assumed it would be a cut and dry process similar to how a 3.5 mm earphones or speaker being plugged into a computer just works. Apparently not; and seems to be a particularly common occurrence.

All this ensued when I was trying to check the sound quality of sE Electronics Laser shotgun microphone that I finally managed to open after many moons. So without much ado: