1. Place a base color, specifically a neutral shade of gray(in the middle of white/black). If you want to have colored metal, it's best recommended to add color at the very end using Overlay.
2. Add a layer and clip it over the base layer. Use a dark shade of grey. Off-black is fine, but do not use black! It can interfere with lineart details such as plates or creases. After placing a line, blur one side only. Leaving a dark line emphasizes a crease in the metal. You might want to add a few more shades around the shape if it overlaps another plate of metal. When blurring, it's okay to blur the other end of the line a little bit to give a full gradient feel as well as a small/faint "dark" glow. Merge this layer and create another layer and clip it for highlights.
3. Set the clipped layer to Luminosity/Add (depending on the version you have), and use white/light gray to add highlights and shine. I added another highlight on the darker side fora shine effect. Like with the shading, blur the highlights to give that smooth-gradient look. Sometimes you'll have to add more whites to make a shine or emphasis on the light source. Small circles, ellipses, and outlines in white would have to be an extra highlight.