When you start adding a new character, choose the owner from the drop down menu. Then write in the character name. Pick the character's position from the drop down menu. Department heads are chiefs, so the CMO would be Medical Chief and COS would be Security Chief. Junior officers are assistants. If you're creating a custom position, tick the override box and type in the position title. For example: Bartender, Xenobiologist, Archeologist. Select the appropriate rank, and press the Write User to Disk button to save your changes. If you make a mistake, press the Reset button, and start again.
Of note: Only players with FComm access or above have access to the ranks above Commander. Also, the Crewman rank is not available in the Create Character mode (for reasons unclear to this writer). If you have an enlisted character, create it as a Civilian, then use the Modify Character option (explained in the next lesson) to change it to Crewman.
Giving a rank puts the character on the roster. If you do not want the character on the roster (ie: GM characters for each sim are kept off the roster), leave the rank area with No Title. If the character has a rank, put it with the name. (ie: Admiral John Doe rather than just John Doe.)