There's a process called brush plating where you just have a swab with gold plating chemical on it and apply it that way. Not a top grade technical job and a little vulnerable to corrosion but nice.
Copper and brass need a barrier layer of nickel over it because otherwise the gold diffuses into the substrate. It's also prone to pinholes creating corrosion.
Chromed stuff is easier you remove the chrome in one step and there's always a solid nickel layer underneath that was put there to support the chrome job.
Steel requires a nickel layer before the gold, but IIRC it needs to be especially thick because the steel's so prone to rusting.
Aluminum involves some exotic processes to plate.