See the components that make up the construction
Solder a 5-1/2" wire to the 3.5 mm jack, see figure 5. To the positive (tip) terminal solder a red wire, to the ground solder a black wire. Once the wires are soldered to the plug, see figure 6, thread the wires through the body of the jack and screw the body down on the plug.
Solder the red wire from the 3.5 mm jack to the Digital In on the pc board and the ground wire to the ground next to the Digital In.