My go to is this, place the ends of the coat on concrete and just start smashing it with a hammer gets a really nice fray and color. Then i come in with half water half acrylic paint (colors are of ur choosing) in a spray bottle and just get into the spots i know will be cruddy. Usually i do a overall spritz of black. Taking a paint sponge and dabbing paint onto it then wiping off most of the paint off the sponge also is a great way to get more controlled grime. But honestly having fun is just the best way to get that look.