I did a quick aging on one of my costumes a while back that was pretty nifty...
First, wash it, dry it and then ball it up so it gets super wrinkly and the washing takes out all the fabric finishers/coatings.
I got one of these
drill bit attachments used for stripping paint to do some overall passes to fray the fabric. Test it out to get the hang of it on areas like the pant cuffs and knees first as those are commonly very worn in a really old pair of pants but use a light touch. I would put either foam noodles or cardboard tube/rolled cardboard inside the legs/body area to help see the areas better and hold them away from the sides/back as you're working on it.
Also get some sandpaper for finer fraying around edges. I wrap the sandpaper around a sponge for better grip, but they also make sandpaper blocks that are a bit more $.
The drill tool/sandpaper needs to be done with a very light hand, and take breaks to check overall effect. You can always go back and take it down more, but you can't put fix too much wear if you go too hard on the fabric.
It would be super easy to go too far if you're using the drill attachment, so if you aren't working with really thick fabric, likely recommend using sandpaper only to make sure you have better control.
Next part, paints. I do aging using three types of coloring depending. Almost black (like a really dark brown black), medium brown or green depending on exposure and either white paint or bleach. If the fabric is medium or dark colored, I use bleach, if it's light I generally use only darker colors, but hit it with a misting of bleach in the high traffic areas. I use a combo of spray bottles with diluted paint/water and mist areas like the main torso and knees and waist, and then I may dunk the end cuffs into a darker water/paint mix. I use acrylic paint mixed with water. I do all this out in the grass, and have a hose handy as it sometimes looks nice to wet down areas and then pour diluted paint or dip ends into it and watch the paint "bloom" up the fabric.
I may finish with a finger/brush spatter and using just slightly diluted paint at that point. Especially if I'm doing any blood spatter.
Finally I'll take a diluted water/bleach mix and go back into some of the areas and mist them to see if they lighten/fade out more to simulate sun/weather exposure or wear patterns. Usually around the knees, waist, seat (and arms/cuff if there are any).
I'll let it dry outside, and then re wash it, and let it air dry and assess if it needs anything else.
