This one confused me a bit, but its no problem if you have missed it.
When forging the Mithril items at the blacksmith's in Bree, be sure to hit the correct buttons as they move past the INNER CIRCLE and not the outta one. So let them move past the first circle then quickly hit them on the second. You will slowly fill up the smaller bar giving you the perfect item.
Do not panic if you have already forged any if not all of them. You can simply go back to the Blacksmith, click on the ones you need and have as many free turns as it takes you.