NMM, as you may find, is a difficult effect to achieve...especially without the use of an airbrush.
I started out by following this tutorial for NMM swords by Girlpainting(starting at 2:00):
As you can see the wet blending technique consists in "pulling" wet paint so you can see through it letting the layer underneath appear when it dries.
Believe me my first trial did not look good, so I then went back to the theory by taking a look at this picture(picture at 2:00):
As you can see here there are different ways to achieve a metallic look on a blade.
I strangely found it easier to do the sword with several light effects(the one on the left) than the simple one and also found colors easier to blend than grey.
I also started using a "ghetto wet palette" (copyright WarbossTae) which consist of wet paper towel in a plastic dish...which could easily be upgraded to a sponge and sulfurised paper
The advantages of a wet palette is that its easier to see how much your paint is diluted and the paint on it stays wet for a while so you can keep working with it.
Hope this helps, and as always happy hobbying!
Edit 2017: Or you could also do it the GW way!