Pull the wire seperation back far enough on at least one side to permit Heat-Shrink to slip over the wire insulation.

Now the soldering. Place the pre-heated iron on the wires on a heat-resistant, flame proof surface. Make sure that the Heat-Shrink is nowhere near the soldering iron.

Let it heat for a few sconds (copper is an excellent conductor of heat), then touch the end of the solder to the wire. If fully heated, it should flow easily into the joint.