here is my method.
take the uniform and soak it in water, wring it semi-dry but damp to the touch
then put the uniform on the figure and scrunch up the areas where high movement takes place such as shoulders, elbow bends, knee bends, and waist area...
once you get a good looking wrinkle pattern, you can start the weathering process
i prefer using crushed pastel chalk for black cloth as paint weathering is usually too extreme and not as realistic from my experience (although i have seen others pull it off beautifully).
take a brush ad drybrush the chalk onto the clothing, highlighting the wrinkle areas.
wehn weathering, keep in mind the areas that would have the most abrasion and wear, usually knees, elbows, shoulders, thighs, butt, etc. and highlight them accordingly
if you have a good 1:1 referrence it makes it much easier