I'm a tinkerer, so I don't have a foolproof, unified way of producing decals and I'm rather happy to see that they are considered pro-material.
It's usually a matter of finding the right source material.
I usually find my best source materials at
http://www.cgtextures.com (see the splatter section for nice sources to make bloodstains, esp.)
I'm very happy with the big brown mud-stains, myself. Here's how I made those:
1. Source:
[lvlshot]
http://www.tabun.nl/tmp/decal_tut/dec_source.jpg[/lvlshot]
2. Then I do whatever I can to separate the stain from the background or at least make it possible to make a cutout without too much fuss.
Here I used Photoshop's "replace color". Select the lightest color in the background. Or actually, somewhat of an averaged value.. I selected the lightest part in the section right between all the stains. Tinker with the fuzziness until the stains get a nicely distinct outline (here I used 65). I also had some sections of stains selected with that setting, but that's ok. Then, I set the lightness slider all the way to the right (or however far it still looks right -- can sometimes get you jagged lines).
Result (crop):
3. Now comes the hardest work. I use my tablet and a varied set of brushsizes and pressure settings to make the surroundings of the stain white. The replace color trick (and any editing you may have done with brightness values overall) really help to hide brushstrokes as long as you're careful. I also disconnected the various shapes of stain where they could, so I could get more separate textures. This wasn't exactly a perfect source image, so it took a while. Better images leave you with very little noise to edit out.
4. Then comes tweaking. The stains are way too dark to use on most of my textures, so I added an adjustment layer and make the whole image brighter. It usually takes a few tries to get the right brightness (and thus, transparancy) to use on your textures.
5. Finally I add an adjustment layer that inverts everything underneath it. That way I can use obsidian's shader, to make the decal look ok in fog.
Step 4 had me thinking that it's probably a good idea to get some variation in the darkness/brightness of decal images in that q3w_decal library, so they can be used on many different surfaces.
I'm not so happy with my "cracks" decals, but if used right (subtility is key), they can work. What you need to look out for are source images that are already as close to crips outlines as possible-- i.e.: dark crackles on very lightly coloured mud or white plaster, coffee stains on clean creaseless paper, dark rust on bright yellow metal, etc.
What I didn't describe above is how to make tiling "trims" like the leaks in my decal set. But I guess everyone knows how to make tileable textures and stains/rust is particularly easy to work with for that purpose, anyway.
(The name's "Tab
un," by the way. ;])