Step 1: Materials and Tools
You will need to buy a switching 3.5mm (1/8") female headphone jack. The important part is a switching style like the one from mouser below. It should have 5 contact points Left + in and out, Right + in and out, and ground. We need it to be switching style because we want to still use the internal mic when one is not plugged in, but switch to external only when one is plugged in. I recommend getting this one or the one just like it with threaded bushing .
You also need about 2" of 2 conductor wire about 18 - 22 guage.
small screwdrivers flat and phillips
small file (not shown)