It really is, just let your imagination take hold. It helped me to look at actual guns and how they weather with time and use. I usually cut small squares of foam, spray some silver spray paint on a piece of cardboard, wait about 60-secs, then coat the foam with the paint. Wait another 60-secs or so, then "lightly" start to spread it out. Try it out on a toy gun or other plastic parts to get the hang of it. Best rule of thumb - less is more!
For the magnet, note the "round" trigger mechanism. I have an earth magnet under it, with a corresponding piece of metal on the holster. It securely holds the blaster in place, even while I walk/run.