With black, it's a bit of a stylistic choice you need to make. If you're going for realism, you should probably not base coat with pure black, but something JUST shy of it. Then you can make black chips in the armor that look like shadows, and highlight the bottom edge of it.
I suppose you also need to decide if your battle damage is fresh, or old, or what the material under the paint even is to begin with. Let's say you imagine your tank is some kind of steel, so if you want fresh damage, just a few brushed (not drybrushed) on paint chips using a silver. Hit the bottom edge of where your silver chips end with your highlight color. Older damage could be rust colored with some rust streaks going down. For dirt and grime, typhus corrosion dabbed where you think dirt would settle works really well. Personally, I am a "less is more" type of guy. I see people really slather on the dirt and grime, and I don't think it looks very good for
40k stuff. You lose a lot of the asthetic, I think, that
40k is about.
If you go pure black, which I like to do with space marine stuff because it's already a bit cartoony, you'll obviously be doing a ton of edge highlighting. Use your edge highlight color for scratches. On edges and corners, draw a scratch with your highlight color, then where it intersects with your edge highlight, make it black. Kind of hard to explain so I'll show you what I did with my Repulsor just last night:
I also used a sponge with black on some of the decals/stripes to show some paint chips.
The way I imagine my vehicles is that they've seen some action, but they're regularly cleaned and annointed with sacred oils by techmarines, so there's no buildup of grime on these machines that are blessed by the omnissiah.
For practice, I recommend you take a spare bit like a hatch or something from a rhino (which most people have if they're building
SM vehicles) and play around.