I’ve done this several times with my CH ignitions and Brison engines..
1 Cut the wires where necessary
2 Strip back the insulation carefully
3 Use a little bit of flux on the bare wires
4 Tin the wires
5 Slide some heat shrink tubing on either end
6 Use a fixture to hold the wires parallel and next to each other
7 Flow some solder on the tip of the iron and wires at the same time
8 Inspect the joint under a magnifying glass
9 Remove any excess flux with alcohol
10 Place the shrink tubing over the joint and heat it

And make sure you get the polarity right. Dan.