How to solder wires on to this power barrel plug?
Tin both wire ends first. Fill the middle cup with solder, and stuff the (stripped) positive wire in while the solder is hot. Push the negative wire through the hole from inside to outside up to the insulation, and solder the wire to the outside of the contact towards the plug. Crimp the arms around both (insulated) wires. Screw the cap on (and oh yeah, don't forget to put it on the wire before soldering anything).