Good question. This is rather time consuming - certainly gives me a healthy respect for the guy who did the artwork for ATGold and DC series (FYI, my background is graphics, so I may have a headstart on some of this...)
I usually look for some old photographic research, archival photos, or even photos of model tanks.
Then I will draw the basic contours of the photo in Photoshop, creating a new layer with only my linework. Then I will eliminate the photographic source imagery, erasing the original image with only my linework left. Creating an original piece of linework art with proper lineweight is the key here, keep your lines pretty thick.
I add color on a layer underneath the linework, being sure to simplify the shading - the final images are pretty small.
Then I open up one of the "FordSFTypes" from the ATGold graphics file. I know these are the correct size, and have a transparent background. I also like to overlay my tanks with existing tanks so the scale and perspective is similar. Same thing with infantry - I'll overlay a Gebirgsjager over the plain old rifleman, to insure similar scale. This will create a new layer in the Ford SFType png file.
Now I erase the original layer that has the FordSFType, so that my layer is the only one remaining. I rename the file based on what it is, say "Gebirgsjager - base"
Then I create a duplicate file with the surname "overlay" - this will be my color layer. "Gebirgsjager - overlay" From this layer you keep everything you want in color, and eliminate everything you want in the regime color. Go back to your "Gebirgsjager - base" file, and desaturate the colors. Make sure your contrast levels are pretty high on this file too, the darks being very dark and the lights being very light.
Then create your own graphics subfile, and they are ready to import via the editor.
Best way to get started is to examine the "FordSFTypes" in the graphics folder for ATGold. Look at his SFTypes, and see how he creates his art. This will make more sense once you do that.
By the way, I plan on releasing my mod on the community site pretty soon, but plan on future releases with more historical SFType graphics as time goes by.