Modification How can I weather clothes?

It depends on what kind of clothes it is and what kind of wear you want to simulate.

First step, if at all possible is to wash the garments. Not only does it tend to let seams and folds to settle a bit and make the gament look a bit more natural, but it will also help any paints or dyes you use to stick better.

Paints (acrylics or fabric paints) are great for building up grime and they can also be used to fake sun bleached areas if needed. They can be used either watered down to let it soak into the fabric or straight to give a bit of a dry brush to pick out areas with wear. You can also experiment with how the paint behaves on wet fabric versus dry fabric.

Dyes can be used in a diluted state to give a sublte all over tone down. But it requires a good idea of the fibre content of the clothes, both fabric, notions and thread.
Bleach can be used to give the clothes a washed out look, but again you need to know what materals the clothes are really made from to know how it will behave with the blech and to estimate if this is a fabric that would be likely to get washed out i the first plpace.

If you have areas that need a bit more mechanical wear, sand paper can be used to rough up the fabric. Don't do this if you have a material that is trying to look like something it isn't, as it may not wear the same way as the material would if it was real. (Fake leather to represent real leather, wool blends to represent pure wool or any other fibre posing as something else.)

The only way to know for sure what would work for your specific garments is to do tests on the same materials as the garment is actually made of. Sit down and play around with the doll with the garment on, to mark out what areas would get worn. Realistic weathering is just as much about getting the right kind of wear in the right places as it is about the techniques themselves.

Last thing to consider is if you want it to look natural in real life or on photos, because you often need to over do it a bit to make it show up in pictures, but that can look a bit fake in real life. It's a preference thing.

 

Really good advice above, and I agree, it has everything to do with what the garments are, their materials, and how you want to weather them.
If you have a natural fabric like cotton which takes dyes easily you can use tea and coffee to create an antiqued or a bit dirty appearance really well. If the fabric is lighter rather than dark, I recommend painting with watercolor over acrylic to stain, since it absorbs into the fiber like a stain instead of laying on top, which simply never looks right to me unless it's an acrylic paint stain look you want. Watering down the acrylic sufficiently is good too, but the colors watercolor comes in (and easy to find) are a more natural mineral based palette and it looks more like real dirt to me. Sandpaper and bleach are also fun to use, if that's appropriate for the fabric. You just have to be gentle with abrasives and always start small. You can also take a seam ripper to pull threads out and create small runs.

 

I bought a pair of cheap canvas doll shoes (the ones that look like Converse shoes), soaked them first in bleach then in coffee and turmeric mixture. I think after that they look pretty used!

I tried the same with a jacket I made out of an old pair of jeans but it didn't work. The bleach didn't lighten the fabric at all.

 

Lots of good ideas on the painting and dying of weathering, but I also use wire brushes, rasp files and serrated bread knives, depending on what I want to achieve.

A technique I use involves jamming a garment in a draw on one end, then taking either the rasp file or the bread knife and just..sawing at places you want to pluck holes into. This is especially good on knit fabrics such as t-shirting jersey.

Sometimes I think weathering only looks right with repeated actual wear, which is why I do it this way.

A lot of my characters have very ‘moth eaten’ dystopian clothes, and those usually require quite a lot of roughing up.

Other’s have said too, it depends on the fabric choices. But it can also depend how and where you cut the fabric. For example, if you want to achieve a really heavily frayed edge you can cut your patterns on the bias. (Presuming, in this instance you are creating from scratch.) this will create twice as many thread ends as cutting along the grain.

I am always really careful about the choice of fabric, both fibre, and weight, as that all plays into the final results. Non natural fibres will be much harder to dye than natural ones. The dyes that dye poly will also dye your doll, they can also stain, even when totally dry, I think it’s important to mention that, and with any dying project it’s always best to stick with natural fibres, but even then soaking it in a mordant for some time can help ensure colour fastness.

Sorry if I’m repeating anything that others have said. Hope any of this is helpful.

 

If you need to see examples of this stuff in action, I would suggest watching Rachel Maksy on Youtube (a lot of times she "ages" her clothing and projects using the above techniques). I am pretty certain the cute mouse felting video had some armor weathering in it, specifically.