To use the .rename feature, hold the item you want to rename, then type
.rename NewItemName in the chat, replacing "NewItemName" with what you want the item to be called.
As for FancyChat, it actually uses a special range of unicode characters rather than a different font style. This is why it can bypass swear-word filters on many servers. However, it does not change the actual font in the traditional sense, which is why you can't customize how these messages will look.
I hope this helps! Let me know if you have any more questions.