How does the Fantasia item work?

[TeraRamis]

I'm going to be subbing soon and when I get the Fantasia Potion or whatever it's called, there are a number of minor things I want to change about my character's appearance.

The thing is, since I only get one crack at this, I don't want to go in blind. How does the item work? Are you allowed to employ previously-saved templates to apply changes (thus giving you a chance to make sure everything is right before even using the recustomization)? If you don't use a template, does it load your current appearance to make the changes on? Or does it start with a clean character?

Understandably, I don't want to spend a lot of time mucking around with the thing on the off chance an internet disconnect screws up everything.