A Perfect Mithril Treasure item is forged at the Blacksmith in Bree. When you get to the blacksmith's shop he will not be there and his wife will give you a quest to find him in a nearby cave. Complete this quest and you will be able to forge items.
In the blacksmith step into the marker and choose which Mithril item you wish to craft. You will need the appropriate design, a certain amount of Mithril Bricks and a certain amount of varying types of loot.
When you start forging you will see two rings. To forge the Perfect item make sure to hit the corresponding button when it hits the inner ring and flashes yellow. Hit enough buttons in the inner ring and you will fill it, which will give you the Perfect item.
If you fail you can try again for free by clicking on the item on the menu
Once you have forged all 32 items perfectly you will get the achievement.