Many writing teachers and critique partners will tell you not to use flashbacks at all.
I’m never one to discard a potentially useful technique. It is possible to use flashbacks, and to do them well. You don’t want to use too many of them, or readers will start to wonder why you didn’t just start the story back then. They absolutely must be necessary to the plot, or why are they there? And they must be written in as engaging a fashion as the rest of your novel.
☐ Flashbacks, if used, are kept to a minimum, are necessary to the plot, and are engaging.
Note that they must be all three, not one or the other.
One place where flashbacks can be beneficial is when you reach a point in a story where some historical information must be revealed, and your initial instinct, in the first draft, was to have the person who has the information relay it to the protagonist in dialog. That’s fine if it doesn’t go on too long. But if the speaker goes on for a page or so, and the telling of the story involves a lot of dialog—so you wind up with quotes inside quotes—consider making that a flashback.
Even dialog can be telling, especially if it’s relaying something that happened Way Back When. Fully dramatizing the scene in a flashback will be more engaging for the reader, because it allows you to show what happened instead of having one of the characters tell it.
Where to place flashbacks
I’ve heard two schools of thought on flashbacks. The first says put them early, so your reader has the information they need to process the story. The other says to delay using them as long as you can, because early in the story the reader needs to be fully engaged with the characters and the main plot before being distracted by the history.
I’m normally inclined to agree with the latter view, but an early flashback can be effective if we care enough about the character. In Rescuing Olivia, Julie Compton uses this technique to good effect. At the start of the novel, Olivia is comatose following a motorcycle accident. The early flashbacks work in that novel because of our sympathy for Olivia and her boyfriend. Because she’s comatose, flashbacks are the only way we can get to know her.
What it really comes down to is that a flashback is only effective if the reader cares about what it shows. If the flashback engages the reader with a sympathetic character and moves the plot forward, if it answers questions the reader is asking, it kind of doesn’t matter whether you place it early or late.