First thing you want to do is compile reference images. You want to get an idea of the overall costume for yourself. Once you have that you break it down piece by piece. For instance, the Graverobber has pasty white skin, wears lipstick and if I remember correctly his hair looked like it head dreads. So you kind of take the character like that you break their look apart to it's bear essentials and tackle the simple things first.
So next you know he had a real raggedy coat right? Well you have two options, you can either make one and distress it or you can thrift shop a coat and distress it. Either way you are going to have to look into wearing down and distressing materials.
Best of luck!
