First, you need to decide a direction of light source (usually from straight above the mini, if there aren\'t any other apparent lightsources) and then paint your shadows and highlights as if the mini were lit from that direction. This goes for the entire mini, not just the areas that are supposed to be metal. What you do with the metal areas then, is to exaggerate highlights and shadows and make sure the contrast between them is high. Take the highlights up to pure white in the brightest spots and make the shadows really dark.
This is the basic idea... you need to practice a lot to get it right. When you have tried it out a few times you will have more specific questions to ask and you will be given more specific answers.
You can also use the forum search and go through all the threads here that have dealt with NMM (it\'s quite many...

) and you will find lots of information there. But, you need to practice it, still.