I use the following things in the following order of application, but I often mix things up and try new techniques.
HINT: Some weathering is done while the bag is wet, some while it is dry. Doing "wet" stains (e.g., sweat) while the bag is wet helps the "stain" bleed as it would in real life into the cloth, leaving a softer edge on the stain. Doing "dry" stains (e.g., dirt, mud, etc.) while the bag is dry makes it more realistic for those stains. "Dry" stains are done last, of course.
- * Extremely, EXTREMELY thinned grey acrylic paint (thin it with water, of course). This is the "base coat" on the white cotton. Wet the bag before this step. You want to cover most of the bag, but not 100% of it. This ages the whiteness of the bag, but still allows for some white to show through, hinting at what color the bag was originally. That's why it's better to start with a white bag instead of a gray one, IMO.
* Strong tea. Wet the bag first, and splatter onto the bag in larger areas. Roughly 75% coverage. Let completely dry before the next step so the stain "sets" in.
* Strong coffee. Wet the bag again, and splatter coffee onto the bags with roughly 45% coverage. Wetting the bag again after the tea stain has set in will sometimes "push" the tea stain out a bit, giving that "sweat ring" look. Let completely dry before the next step so the stain "sets" in.
* Are you ready for this? My secret ingredient: Red Man chewing tobacco. Boil it down in some water in an old pot, or your wife will kill you. Wet the bag again, and sprinkle this onto the bag. Don't do much, just a little. This will give a little more "red" tint to the stains, since everything else is a shade of brown. I'll also put some of the actual soaking wet tobacco clumps INSIDE the bag down at the bottom to stain from the inside out.
* Finally, let the bag completely dry, and rub some mud or earth into parts of it. You want to wait until all of your "wet" weathering is completed before doing this step because a lot of it will wash off if you get it wet.
Wayne