Plan out what wires need to be spliced together, double check with your Harness Documentation that all the wires have been identified.
Trim all the wires to be spliced together to the same length, and strip the appropriate amount of insulation off the end, typically around 5mm, but will depend on the splice terminal used
Lace all the wires to be spliced together using a Double Surgeons Knot.
Crimp on your splice terminal.
Apply Heat Shrink Tube to the splice, make sure to place the splice only half way in to the tube before applying heat to recover the tube.
note: some people like to crimp the end of the hot heat shrink tube to seal the end. I don't like this method, I have see tube open back up under high heat situations and "un-seal" it's self.
Apply a second piece of Heat Shrink Tube to the splice, folding over the first tube to create a sealed end.
Position the top Heat Shrink Tube neatly over the bottom and apply heat to full recover the tube.
Using the lacing cord that bound all the wires to be splices, secure the splice to the wires. be sure to give your self some extra forgiveness wire just in case you want to add more branches later.
A small explanation of this great
feature, in clear words.