Some people should be kept away from wires.
Remember solder is to make electrical contact not a mechanical joint.
Best way to join wires is to splay the ends, push the two ends together so the strands intertwine and twist. Then apply solder and cover with heat shrink tubing.
If the strands aren't shiny, use wet n dry over them to remove oxide.
Alternatively use a butt join connector, I don't like them as they are more bulky.