You create a character profile here, fill in the necessary information and stuff given in Character Sheet sticky, and then put it up as a new thread in that section.
Once that is done, you can go directly to the Sith sign ups here and fill in the small sign-up sheet.
If there is anything that needs to be changed about your character, it will be mentioned by either one of the faction leaders of the Sith Empire or by a member of the community. Although we do not have an approval system, we do expect changes to be made to the character if it is deemed out-of-hand by multiple people. If any changes are needed to your character, they have to be done before you can begin role-playing.