I don't use decorating items because of the fact you can't directly specify what you want decorated, which is annoying since my dwarves do end up decorating useless barrels and bags. I know there are ways of making them decorate a specific item, but still
Decorating is easy: Make the barrels and boxes storage stacks for trade goods only take stuff of lower quality, and put the decoration area far away from the rest of the fort (I tend to put the decorating chambers east of the main staircase, down a long wide corridor, while the rest of the fort is west, or vice versa. Then fill that wide corridor with glass and gems and other good things. The far end gets a storage area for stuff you want decorated, bin and barrel free, and a jewellers and so on. Then that stuff will get decorated, and the bins and barrels won't. And when it comes time to sell it all, it's not hard for a bunch of dorfs to come pick up the good stuff and take it to the trade depot, enjoying beautiful long wide halls of decorations while they do it, giving them happiness.
Stuff often arrives decorated, too. The stuff with wooden decorations is the worst: Somedorf keeps putting the tree seeds (apple, pear, bayberry) in bags that were purchased pre-decorated with hanging wooden rings and the like, making it really dangerous to trade bags of seeds to the elves.