Heat Shrink Solder Butt In-Line Connectors 1.5 to 2.5mm² Blue Pks10+

Strip back cable insulation
Insert wires into each end of the In-line Solder Butt Connector and twist around so that they interlace with each other
Heat evenly around the solder ring situated in the middle of the In-line Butt connector using a small gas torch flame
When the solder has melted evenly into the wires, move the torch around the remainder of the heat shrink until it has shrunk to its minimum. 

Use TT95 Micro Torch to solder/shrink these Solder Butt Connectors.