This will help you out:

  • Log into your SE Account!
  • Under Services and Options on the bottom Left, choose Select Service
  • Hit the text or image of FFXIV now on the main page to the right
  • On the next screen under Active Service Accounts, choose Options List
  • At the top of the next screen is Character {how many slots avail.} and "Add New Option" ~ select that
  • On the next page is a FAQ of charges, read it if you like or select Next at the bottom
  • This next page is a drop down box to select how many characters you want to unlock with the Crysta fee listed, but don't worry about that as there is no charge for anything currently.
  • Choose one or more and select next
  • And on the next page it'll detail your selection with a red "Add Options" button to finally confirm a new character slot.
  • The final screen will be the same with a red "Done" button that will return you to your overall account details page.