To start writing your 'Mass Effect' female OC fanfiction, begin with the character. Consider her skills. If she's a combat - oriented character, what kind of weapons does she prefer? Then, you should think about her relationships. Who will be her friends or enemies in the story? For the setting, you can choose a familiar location from the game like Omega or the Citadel. Once you have these elements in place, you can start writing the opening scene, perhaps with her arriving at a new place or getting a new mission.
First, you need to create your female OC. Decide on her species, her background, like whether she's from a military family or a civilian one. Then think about her personality traits, is she brave, shy, or something else? After that, you can start thinking about the plot. Maybe she could be a new recruit in the Alliance and gets involved in a mission that ties into the main Mass Effect story. For example, she could be part of a team sent to investigate a new Prothean site.