You basically create a height map. 50% gray is flat, black pushes in, white sticks out. http://img521.imageshack.us/img521/9046/normalmapminitutrf7.jpg has a good example
Thanks for the responses! @Sean_EG The most important detail of what I'm going for are grooves to create the illusion of separate armor plates. I do in fact have nDo and am pleased with what it's capable of, but I'm not sure how to get a "negative" shape in it. Ndo is great for sculpting raised areas, but I haven't figured…