Old article floating around for years, which I first saw featured on MAKE magazine (2012), outlines how to solder two solid core wires to NASA standards.

The technical specification document in question is NASA-STD-8739.4 - WORKMANSHIP STANDARD FOR CRIMPING, INTERCONNECTING CABLES, HARNESSES, AND WIRING (2016-06-30)

Screenshot NASA-STD-8739.4A – 2016-06-30