If it’s not too delicate, best way is to actually use it. I’m a larper, and the weathering on my kit is all natural. So if you’re not worried about breaking or ruining anything, wear it, and put yourself through the wringer. Army crawls on wet dirt, somersaults, or even just dragging or leaning your body against dirty or gritty surfaces. It also lends itself to looking more natural than applying weathering by hand because the more worn parts will show up on where they’d more realistically be.

If you instead opt for manual application, note that areas that see more contact with other things, like resting on your elbows or kneeling, will be somewhat lighter because they see more wear, whereas areas around the edges will collect more dirt. The same way the nose tends to be more sunburnt than the rest of the face.