Well, it's not step-by-step like the other folks do ...but.... here's how I did it:
1.) Paint the smokebox and stack a flat black.
2.) Take a tube of "lock lubricating" graphite (about $ .50 at your local hardware store) and squirt out a pile of graphite powder onto a piece of paper on your bench.
3.) Using the still wet brush from the flat black paint, scoop up a bunch of graphite, and slap it on the smokebox. Work it around, until all of the area you painted flat black is covered with a pastey graphite .... you can squirt powder from the tube directly onto the locomotive as you go; the parts you haven't painted flat black won't hold it unless you rub it in, and it can be blown off if you spill some where you don't want it. Using the damp brush make sure everything is covered.
4.) Your brush will dry out at this point .... generally from being covered with powdered graphite. Using it, and if you like, a larger soft brush, you can now polish/burnish the whole smokebox area until the semi-shiny graphite look is about evenly distributed. Be prepared to go over problem areas until you can make it somewhat even.... but remember, brush marks and a texture of sorts are PROTOTYPICAL. This stuff was generally applied to real locomotives with a really big brush, or even a broom! It helps if you have a cradle or other arrangement so that you can rotate the locomotive so that you're not trying to apply the powder to a vertical (or over vertical) surface.
I am working on a second version of this that's closer to how we did the real smokeboxes ..... the real shops mix graphite powder with boiled linseed oil to form a kind of paint, and apply as described above. While linseed oil will eat plastic, there are several light oils that will not, and I'm going to try to come up with a mix that can simply be painted on and polished just to save the powder all over the place. But... #8 looks great, I think.
Here's another one I'm working on (not a great photo, but you get the idea ... I need a real camera!)
What will be really neat will be what #45 looks like once treated, and given a round number plate (That's the K-27...)