Situate your flashback after a powerful or significant scene in your novel. Flashbacks are always triggered by significant events.
Explain to the reader where and when the flashback occurs. Make the start of the flashback clear to the reader to avoid confusion. Inform the reader of the year or age of the character having the flashback and where she is when the flashback happens.
Signal the beginning of a flashback by using the past perfect verb tense ("She had been"). Most stories are written in the simple past ("She was"), so when the writer switches to the past perfect, he informs the reader that the story is traveling back in time. However, during the actual flashback, switch back to the simple past again:
"She had been here before, somehow. She had been here before, but she wasn't wearing these clothes. She had been wearing a red dress, and she went over to the man in the black suit...."
Tell the flashback story briefly. The flashback itself should contain a significant event, usually something traumatic -- something that the character doesn't normally remember or want to remember. A flashback memory is so powerful for a character that it hits her full force, whether or not she wants to recall it. However, stretching out the flashback too long can cause the reader to lose interest in or even forget about the broader story.
Transition from the flashback back to the story by describing the impact it has had on the character. Because flashbacks usually recount trauma, the character should be emotionally affected by such an experience.