This tutorial will be primarily about working primarily with glossy blacks and NMM, though the techniques could be applied to most high-contrast techniques and I'll make a few notes throughout on how this can be adapted.
This also covers a few of my preferences on paints and techniques.
I'll be using a Studio McVey Sedition Wars mini as the basis for this demo.
Option- other colors: If going with a different color, just try to start with a midtone. White will make too strong of a contrast and dark colors won't give any volume. If you've already primed the mini white and are painting a base color, thin the paint slightly so it gets a little initial volume since you're already putting down another color.
2: Wash (dark grey)
This'll be largely done by feel, but you're looking for something that stains the whole surface, and won't entirely rub off with your finger if you try to wipe it after a few seconds. Too thin, and it won't do anything; too thick, and you might as well have started with a darker shade.
Paint the whole surface you care about this color.
Note: I prefer dropper paints (like Vallejo's line) over lid tops (like P3 or Citadel), because when mixing or thinning you have more control over the color and don't need to expose the paint to the air or scoop it with the brush, meaning it's less wearing on both paint and brush.
3: Wash (black)
This is to smooth out the blending further, with a second, dark layer, thinned pretty heavily- you're really only going for the cracks, here. This mixture should be about as thin opacity as milk.
This is still just getting the basic recessed shading down.
Option - other colors: Again, if using warm colors, mix a bit of brown in instead of straight black.
Option - simplify: You can just start with black (or dark brown), to speed things up, though it doesn't get as smooth of a shade.
Option - lighter metal: If you're looking to go with a middle-dark metal (like the Astral Claws featured in my blog), you skip this step entirely, and start blending lighter immediately, or selectively go for the darkest areas.
4: Soft Highlight (dark grey)
This is the first step where you need to consider the angle of light: For basic NMM, you'll be looking at getting a basic highlight against the upper surfaces or to contrast with shadows. Areas you should hit are any larger muscle group/round surface (thighs, shoulders, calves, knees, gun barrels, helmets, etc.; in this case also the collar) and edges (the tops of weapons and armor plates).
For much of the highlighting, I use paint that's a little thinned, with the edge rather than tip of my brush where possible. The reason for this is, it will naturally pick up the edges and I'm not needing to be that specific, so there's no need to wear on a nice brush tip.
Note: Some areas are going to be ignored in this demo, since they're getting painted with another color (such as the shoulders) - check the top image for reference if something isn't looking right.
5: Soft Highlight (mid-dark grey)
Notably exempt are the harder edges, such as most of the angular bits of the armor. You're looking for a stronger contrast, so the edges don't really need to be included in this step, since they'll just be painted over, later.
This is around or slightly darker than the base coat.
Option - dull shading: For some material, like neoprene, dull stone, or fabric, it might make sense to stop here, since not everything needs strong highlighting.
6: Highlight (middle grey)
Unlike the last step, this does include hard edges, as too sharp of a contrast will still look overly artificial.
In this case, the gun, butt, hands, toe of the boot, and calves are the focus, though the shoulder and back would be too, if they weren't getting painted over.
Option - contrast: If going for a high contrast, but not glossy/metallic, surface, this could be a good point to stop.
Option - other colors: If you don't have lighter colors prepared, mixing bone to cool colors or yellow to warm colors tends to get cleaner and more natural highlights, with grey if going for dull and white if going for pastel.
7: Highlight (light grey)
These are just for the strongest edges, like the top of the gun and the tip of the boot, and the top edges of the thigh armor for contrast with the under armor.
8: Highlight (white)
9: Other colors