Thread the rubber/plastic stopper back onto the wires.

Do this AFTER you have applied solder to the tips of the wires.

To do this, I sprayed a bit of alcohol on the whole thing, and pulled the wires with the pliers. It required a pretty decent amount of force—but don't worry, the cables should hold without a problem. Just make sure you're pulling both at the same time.

Note that I've also added a tiny length of shrink-tube on the exposed wire.