Part 2 of the video about painting a marble effect on buttercream shows how to do the detail and the gold on the cake, with a few different methods to do the gold. This is another long one, but it has a decent amount of information in it. Part 1 was published last week, and is also on my youtube channel.
Remember that the gold that you use needs to be edible. There's also some controversy about whether gold leaf is actually "edible" or non-toxic, so you need to decide about using that for yourself. Whatever you choose, let your customer know what the color is made of and let them decide whether to eat it or avoid eating it.
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.biz and www.acaketoremember.etsy.com