To tin a wire, strip about 3/8" of insulation from the desired wire, twist the strands of exposed wire together, and apply a small amount of solder so it flows between the strands of wire. The tinned wire should not be noticeably larger in diameter than the bare wires, and there should not be any "blobs" or "spikes" of solder. Practice on extra wire till you get the hang of it. The shape of each individual strand should be visible through the solder, and no exposed copper should be visible.