Here is a good vid that covers the important parts, good and bad that you want to look out for, the only change I would make is do EVERYTHING with 63/37 Eutectic solder, unlike 60/40 it has no "plastic" state so it is almost impossible to get a cold solder joint. But if all you have is 60/40 that is fine, just pay extra attention to how full of solder, and how shiny the joint. I would also likely double heat shrink if I can and don't forget, the wire will not be malleable near the joint and to bend it there will weaken it significantly so you may need to trim some wire to get your joint closer to the motor so that you have the flexibility to form it near the ESC. Use the same gauge and similar strand size as the the motor already has. It ain't the prettiest but the practice is sound, and with care you can make it pretty nice, and you don't have many choices if you want to reuse those motors. This vid is not about quad motor wires, but the principal is the same regardless.