I can help! Learning 'breakdown' was my favourite thing when I was in costume class at school. :3
For tattered clothing:
- use sandpaper and your own hands to create realistic-looking tears and rips. Scissors create too 'clean' a cut, so use them only to create little holes to start where you want the damage to be, then just RIP as you need.
- if you are VERY CAREFUL, you can also use a lighter to actually burn clothing. Not recommended if you're working with cotton because fire EATS that fabric right up. Polyester and wool gives a good look and the flame spreads slowly and eventually goes out on its own, but the smell of these fabrics being burned isn't pleasant.
- if you want a burned look without the risk, rip the fabric in a random pattern, and then do a thin drybrush brushstroke across the tear with a dark brown acrylic paint.
For dirt, blood, stains:
- TEA. Use any store-brand black tea in baggies to give things a nice overall old yellowed stain look.
- dirt and grass stains can be done easily with green and brown acrylic paint. For the green, it's best to dilute with water in order to give it the realistic faded look.
- blood can be done in many ways. If you want fresh-looking stains, corn syrup and red food dye is never wrong. It would also be a good idea to look at pre-made fake blood at costume stores and experiment with that. I use this stuff
and it works great on clothing for permanent stains. If you want the blood to look crustry and aged, strokes with, you guessed it, acrylic paint works as well, alternating between drybrush strokes and globs in a darker, rusty colour.
- if you want the stains to look like smears, you can go to town with a big wet paintbrush or even finger-paint. For splatters, lay down the garment on newspaper or tarp or a surface you don't mind getting dirty, load up a big paintbrush with either the fake blood or dirt, and flick the brush just above the surface of the garment. The droplets and spray from the brush will splatter pretty realistically. Use more paint globs and more aggressive flicking motions for bigger splatters.
- if you want to get really gross, you can use cut up pieces of sponge and soak them in paint or fake blood to create chunky textures that emulate splattered brains or even vomit. You can use your imagination with colour and consistency on that one.
Hopefully this helps, good luck!
EDIT: Just in case I sound like a crazy lady, here's some examples of breakdown and weathering I've done on costumes:
A buttload of bloodstains
General wear and tear from enduring the elements
Dirt and breakdown from intensive combat