I don't know any specific tutorials, though I'm sure others on the forum will be able to point you in the right direction.
But when it comes to weathering clothing, I would not suggest putting actual dirt or any 'organic' materials on it, simply as that could cause mold issues later on.
While I am not super familiar with 3A style, from looking it up just now, I would suggest using some kind of model paint like Vallejo or something similar which is very finely pigmented and formulated specifically for smaller scale figures, and mix it with a fabric medium. I would use this kind of mixture and then drybush in the various colours that you want.
For a TWD character that I did recently, to make her look 'dirty', I just used shades of dark grey dry-brushed onto a pale coloured t-shirt and jeans, using a flat brush. For the 'blood' spatters, I used a mix of reds, browns, blacks, greens, etc. For those, since they were stains, I did not dry brush them, but applied them wet with a rounded brush. Any other types of stains on fabric should probably be applied with a bit more water. But on something like boots, I tend to make them look dusty by using very light dry-brush applications of an off-white colour, again using a flat brush. I use off-white or warmer colours for weathering leather, but use greys for weathering metal.
You'll need to figure out what colours will work best on the pieces of clothing, and apply them sparingly. If you have a white shirt, you would decide what colour you want the 'dirt' to be, and then use a small amount of paint mixed with fabric medium, and thinned out even more with water. Use a flat edged brush, and dry it on a paper towel so that there's only a bit of almost dry paint on it. Then apply to the areas you want to look dirty. Add small amounts of paint to the tip of the brush as needed, but always dry it on the paper towel first by moving the paintbrush back and forth like you are painting it, if that makes sense. Some areas on the clothing might require multiple layers to make the weathering look 'heavier' -- I tend to allow the first layer to dry first before going back and reworking it more. If you aren't sure how to apply the painted 'wear' to the clothing so that it still looks good and artistic, find an existing 3A figure that you admire, and emulate the way the paint is applied as best as you can until you get the hang of it. : )
Sometimes it helps to use a spare figure body as a 'mannequin' and actually apply the paint while the clothing is being worn. This helps you see where the folds of the fabric will be, and you can therefore make the wear look more realistic. But it can be done just laid flat as well, of course.
Once finished, allow the painted pieces of clothing to 'cure' for a couple of days to a week.
If you want to make actual rips or tears in the fabric, such as jeans, make very small cuts with just the tip of scissors in the areas you want there to be holes. Then gradually 'open' the holes by 'sanding' them with a metal nail file or something similar. You can gently open the holes further with the tip of the nail file if needed.
Apologies that I cannot show photo examples at this time -- it's very late in the evening where I am. But any questions, please feel free to ask! I'm not an expert but I'm obsessed with painting fabric, lol.
