A flashback is a writing technique that gives the author a tool to analyze a character's motivation. The method allows the author to play around with the time frame of the literature and bring to a story all the elements necessary for a full description of events. The reader will learn about the character and what leads to the way he acts.
Set your flashback at least several pages into your book. If the scene happens too early, the reader will feel like he is reading two introductions. Establish the character to build the reader's curiosity about what led the antagonist to act in a certain way.
Start your flashback after a pivotal scene. Creating drama and then following it with a scene about what led the character into the situation or why he reacted that way will build your reader's interest in the character.
Use flashbacks sporadically throughout the text when the reader should know the character's motivation to act. Time the flashbacks so they are fairly consistent throughout the narrative, rather than in one or two large clumps.
Use a letter or journal that a character wrote as a flashback. These items are often personal and will give the reader an in-depth idea of what the character is thinking.
Use the flashback at the end of the narrative by announcing that you are going to tell a story about the past. This technique frames the story and usually reflects on a time earlier in the novel when the character was different.