Really it comes down to breaking the chest down into shapes, and then just using basic/fundamental shading techniques from there. The main chest is comprised of something like half a cylinder sticking out from a box/rect prism. Going further off that, you can break down the brace thing into several thick disks and prisms. Then you can just shade the planes different colors and decide what to do with cast shadows and ba-bing, instant chest. Ok it isn't quite that simple. Hopefully the process gif shows a bit more. Note I only used 3 tones here so some of it can become befuddled XP