In order for you to make a background of your own you need to:
- The languages that your character knows are determined by the location your character comes from and what their background entails.
- Pick two skill proficiencies you are proficient in that are related to your background (in terms of narrative).
- Pick 1 proficiency other than a skill proficiency that is also narratively related to your story.
- Make up a non-combat and narrow enough advantage that allows you to do something probably only your character can do. (for example: expertise on history checks to learn something about weapons and armor because you were taught by the best weaponsmith of the land)
- Confirm with your DM that everything is good to go or work with your DM to make it up.