Write a comprehensive outline or list of meaningful situations and events that have occurred earlier in the lives of your main characters. None of that information will be known to the reader until you reveal it with the flashback device.
There must be a reason to use a piece of the character’s past. Something has to trigger a memory, good or bad, before you put it into the story. Once you’ve established the need for a flashback, it can be written as a brief moment or a longer return to an earlier time.
For example, "Jennifer studied her image in the mirror and touched one of the earrings she had just put on. 'Stephen … I’ll always love you.' The thought brought back the candlelight dinner and their last moments together.
Her Husband spoke from the next room, Martin’s voice snatched Jen back to the present."
From here, the present time moves forward and leaves the reader with a taste of a secret that other players in the story aren’t aware of.
Assemble all the pages and notes of your main character’s outlines. Highlight one or two significant situations or events for each of those characters. You will be using that information to take your readers into a longer flashback. This longer trip into the character’s history will become past-present. That means any dialogue exchanges or background details will be written as if the flashback sequence is in present time.
Look over all your notes and put them back in order. Now extend the flashback sequence by adding to the character’s memory.
For instance, "Jennifer studied her image in the mirror and touched one of the earrings she had just put on. 'Stephen … I’ll always love you.' The thought brought back the candlelight dinner and their last moments together. It was four years ago when Stephen gave her the gold earrings.
Stephen grinned, poured more Merlot into each of their glasses. Jennifer buzzed with the warmth of the wine she had already drunk. Steve was up to something. Her heart raced with the touch of his hand and constant compliments.
Jennifer smiled, sipped the red wine and her face flushed.
Stephen took a small, black box from his jacket pocket and opened it.
The gold earrings gleamed in the soft candlelight, they were beautiful. Jen promised she would wear them every day while Stephen was away; she never saw him again.
Martin called from the next room. His voice snatched Jen back to the present."