Create a timeline of the significant moments of your character's life. Like many authors, you can use post-it notes or a big whiteboard to visualize your character's life. You can easily move or add events until you feel your character has a well-developed history. After you've finished the timeline, distill it into the top 5-10 moments that have shaped your character. For instance, if loss is a thematically important part of your book, perhaps a significant part of your character's past is when they lost a grandparent as a child.