This is done by first setting the item down in the right spot, then using scrolls to create a barrier around the item. After a few turns, the barrier will break down and the item will be ready! And you'll probably be dead anyway, but the item will be there just in case you aren't.
Famitsu has screens showing off this process, as Shiren turns his sword into a Dark sword. It looks just like it did, but, it's got like a different icon on it in the menu.