Images from Giphy.com. Shot of both official versions of Commander Sheppard from Mass Effect 2.
Choose Their Gender and/or Sex
Perhaps obvious, this can be either important or unnecessary to your story, especially since strange races may remove the need to decide sex entirely, and not all stories go into the concepts of gender.
As with before, choose and write your character with respect to the chosen gender and/or sex to keep from isolating potential fans.