NMM is an application of the way traditional artists and illustrators depict metal objects in their work.
On this very figure, you'll notice that a single blade presents two opposing gradients, going from a very dark blue up to white. The trick is to get as smooth a transition as possible between the two of them.
Specific execution depends on the paints and tools you use, but the real kicker is to figure out how the reflections are supposed to look.
Although, there are some basic principles I consistently try to follow:
1.- Always go from very dark to very light. Color choice doesn't matter, as long as the contrast between the darker and lighter end is very dramatic. Be wary that for the illusion to work, you should always pick colors that have the same base (like, you could do the smoothest gradient using blue and red on opossite extremes, but since those are practically different hues, you'd end up with a convincing "magical sword" instead of a regular NMM one)
2.- Treat each plane with a separate gradient. A plane is a surface that faces the same direction. On this guy, again, each blade has two planes on each side, one pointing slightly up, and one pointing down. Therefore, it has two dark/light gradients per side, running in opposite directions. The directions were chosen more for aesthetic considerations than real physics, but as long as you are consistent throughout, there should be no problem.
3.- Cilindrical shapes always have the reflections painted lenghtwise. It is a common mistake to treat curved shapes the same as flat surfaces. Curved surfaces require observation of real world examples, and then some imagination to adapt that info to the model at hand.