My painting guide is many pages long and would take far to long to type up since it is all hand written. However I can do a few tips bere.
Thinning - thinning your paints is extremely important with this paint scheme, it not only gets you a smoother coat with lighter greys that can be a little thick to work with and might give a uneven covering, but it will give you a better gradiation in tones and shades. Also work in 2 or 3 thin coats for every level you are painting. To thin, 4 parts paint to 1 part water or Lahmian Medium is perfect
Flesh - Causasian/white people will have a paler grey skin tone, Indian Asian have one a little darker and those of African descent will be mid to dark grey depending on how dark their skin tone is. Someone with a coffee coloured skin will be a mid tone, while someone like Tyreese who has very dark skin will be a very dark grey with the lightest highlights being a dark to mid grey. When painting highlights on faces, do so on the nose, brow bone (eye brow area), the middle of the forehead, cheak bones, chin, upper lip (if you are steady enough with your hand) and jaw line. For other parts that are flesh look for things that would catch the most light (true for everything you are highlighting) so don't highlight under the chin, but you can highlight the neck, also never highlight underneath a arm, just the sides and then the top with a lighter shade again.
When working on the model do all of the base coat shades before moving on to shading.
Colour tips - If you look at colours and how they look underwater, they are mited and often darker, this is a good source for how colours will look in a black and white scheme, but you won't be doing them as darl. For instance, red can look almost black at times in deep water where less light reaches it, but red can be many shades. Bright red and be a mid tone while a dark, dirty red can be a few shades lighter than black (and we are talking in shades like you find on a computer where you have millions of shades of gray!).
White - very light grey.
Yelloe - same as white.
Orange - light to medium grey depending on the tone.
Red - see above paragraph.
Green - medium to dark grey.
Blues - very lighthpale grey to very dark grey.
Browns - pale to dark grey.
You get the idea. the best way to convert a colour to balckkand white is to do so on the pc! Get a photo and turn it to greyscale on photoshop or whatever you have access too, even Paint on Windows can do this effectively. Print off a bunch of photos with the original colour one on one half of the sheet and the black and white on the other half (so you can see them both on the same face without having to turn it over). You don't need to do that many, but I have about 50 or 60 of those in a folder on my shelf where my monochrome guide lives (heh, told you the guide would be to big to do in full here).
Shading - once you have got down all of the base colours/shades, thin down Nuln Oil to about 3 parts Lahmian Medium to 1 part Nuln Oil and cover the entire model with it! This is why you do the initial base colours all at once, it saves having the mix the thinned wash again and again. If you can and plan to do a lot of minis in this style, mix a whole pot or dropper bottle of this mix
First Highlight - reapply the base colours to their original places on the minis, leaving the shade in the smallest recesses, you don't want to much shadow.
Second Highlight - If you have a grey that is slightly lighter than the base colour then use that on most of what you recoloured after the shade, if not then add a little of a lighter shade of grey to it. Paint this on to the higher points of the areas, leaving a little of the base colour toward the recesses and bottom of those areas, look at how the shadows fall in those photos you printed off, they are useful for more than just reference for shades.
Third highlight and so on - this is all entirely optional and in fact the 2nd highlight is optional too if you are happy with just the first highlight layer. Each time you lighten a shade use a even lighter grey to lighten the paint, but never mix in white unless that area is to represent white. White is only ever used to paint the most extremely highlights over a very light grey and in small ammounts.
I hope that helps. My full guide (I took a break while typing this post to make sure I covered the basics) has over 150 pages of notes, photos, print outs, greys painted only different materials like paper, card, plastic, resin, etc to see differences in the same shade used on different things (basically colour swatches) and other things all kept in a card folder. It really is a bit of a mess, but it's full of things I have learned over the years and they end up in the guide. I have many guides too, but I need them since I paint at least 500 models a year of my own stuff before counting things I paint on commission and remembering every technique and colour match is impossible, lol!