In the main menu, navigate to WORKSHOP, then CHARACTER to bring up the character creator.


Editing your character is pretty straightforward.


Once your character is done, select upload and confirm.


Lastly, exit the character creator and go to LIST section in the WORKSHOP.
Go to LEVEL, change the option in the dropdown menu to ROLE, and enable your characters.


With all that done, your custom characters will now show up in the Heritage Shop in Initial Adventurer selection when starting a new game.


Note: I have not found a way to change their starting manual; they all start with a greatsword and Flaming Sun manual, so I recommend picking up few manuals for them in the Heritage Shop also.
(I also recommend letting them get to lvl 30 and rebirthing them 1 time while you work on getting your Leviathan level to 5 to unlock the Enlightenment Tower. That way they can "start" with 2 manuals in their proper class as soon as possible)

Hope this was helpful, enjoy! ^^