Flashbacks in fiction are simply scenes from the past. If a story begins at Point A and finishes some time later at Point Z, a flashback is a scene that happened before Point A, usually many years before.
Notice the word scene...
Do you need to use flashbacks in a novel?
Absolutely not. In fact, if you can tell the story without them then so much the better.
You see, what the readers are really interested in is the present story (which runs between points A and Z). Anything which interferes with this is a distraction.
So if the episode from a character's past can be told in a few lines of exposition (telling it, not showing it) then that is what you should do.
If you have no option but to use dramatized flashbacks in your fiction, here are three things you must do...
Have you ever read a novel and somehow missed the fact that the author has moved back in time?
There you are, happily reading about a character in present-day New York, say, when all of a sudden you are in Paris in the 1960s - and you can't remember how you got there. You have to read back to find where the transition took place.
Don't let that happen to your readers.
An obvious way to overcome the problem is to give a flashback a chapter all to itself, and to make the time and place crystal clear at the start of the chapter.
If a fresh chapter isn't desirable or even possible, you have to make doubly sure that the readers are aware of the time and the place - both when you move back in time and again when you rejoin the present.
However interesting an incident from the past might be, it still represents a disruption to the story currently being told.
(I know as a reader that I always groan a little inside when the writer moves back in time, and then cheer a little inside when they return to the more exciting here and now.)
That is why you shouldn't use flashbacks in writing at all if you have no good reason for doing so.
But if they are necessary, it is vital that you don't move back in time unless the reader is hooked...
Ideally, don't have a flashback until you are at least 30 pages in, and only then during an exciting part of the story when the audience will be itching to find out what happens next.
The only exception to this would be if you have so much material from the past that you decide to write a dual-story novel, with one story happening now and the other 30 years ago. Here, you might start in the present, move back to the past for the second chapter, return to the present in the third, and so on.
Forget about fiction for a moment and think about the real world. What happens when we experience memories from the past in our own lives?
How are they caused?
Usually, memories are triggered by an object - an old photograph, perhaps, or discovering our favorite childhood toy in the attic.
Sometimes, they are triggered by our senses. We smell a salty wind or hear a blackbird tapping on glass and we are immediately transported back to an incident from our pasts - an incident in which we smelt precisely that smell or heard that same sound.
(These are known as "sense memories".)
Using objects or senses to trigger a memory of a past scene are precisely the devices you should use to trigger flashbacks in writing.
Say that you want a character to remember something about his mother...
Let's say you choose the apron...
He finds it in the drawer and this triggers the flashback. You can begin it something like this...
The last time Fred saw this apron, he was helping his mother bake gingerbread. He was twelve then and his mother had six months to live, though neither of them knew it that morning...
You can now launch into the scene from the past itself, writing it in precisely the same way you would write a scene in the present-time story, except now you are looking through a boy's and not a man's eyes.
(Incidentally, it is best if these scenes are mini-stories in their own right, complete with goals and conflicts and resolutions, and not just a mass of dull-but-essential information.)
When the dramatized scene from the past is over, return to the present simply by showing the adult Fred standing by the chest of drawers with the apron in his hands.
I can't finish this article without pointing out that, sometimes, almost an entire novel can take the form of one huge flashback.
If a novel begins with a man being led to his execution, for example, and ends with his death a short while later, the bulk of the novel could be him remembering his life story.
Here, though, the flashback essentially becomes the novel's central story (or the "here and now" story), and what is actually happening in the present (the execution) serves as little more than a framing device.