I usually don't do this, but I think this is a really good article on how to do True Metallic Metals. For the most part there are two ways to approach painting metals. True Metallic Metals (TMM) or Non-Metallic Metals (NMM).
Non-Metallic Metal is a process brought over from other art forms where instead of using metallic paint (paint containing small metal flakes), you paint using normal non-metallic paint. NMM is a fairly difficult technique to master and looks fantastic when done correctly, but unfortunately it looks really bad if done poorly.
True Metallic Metals is a process as varied as anything in miniature painting. It can be as simple as a metal base coat, a wash, and then a highlight, to the process in Monkeyman7x's article. As varied as the technique is the results. The results from TMM is not really any better or worse than NMM, both look spectacular when done well, but TMM has the benefit that when used by less skilled painters the results are better,
Anyway, here's the link to the article. I have seen others before, but I think he does a good job of breaking it down into steps that anyone can manage and recreate.
Monkeyman7x's True Metallic Metals