The best advice I can give for flashbacks is to not use them.
Now, realistically, I use flashbacks myself lolololol Like in the case of my current novel, the first chapter is a "here we are" and then the next dozen or so are a "how we got here". I think Thank kind of flashback is fine, where it plays out naturally.
What feels unnatural and inorganic to me, or takes me out of the experience and reminds me I'm reading a book, is when a flashback occurs as one of the following:
In the middle of a chapter
In all italics (seriously please don't do this, long strings of italics are so hard to read for real
To explain something that could also be explained through conversation
I don't mean to discredit some of the advice above me, because you should do what feels natural and right for your story, but I personally don't think (if you absolutely HAVE to use a flashback) that one should indicate the flashback is happening in any way. I'm talking none of this
or this
*~ *~ *~ *~ *~ *
And especially not
begin flashback
flashback text
end flashback
I get this advice from a writing guide I spent much of my early writing years poring over. It's good stuff! The guide effectively says not to use flashbacks. Its reasoning is that the story can be told in a much more clever way, one that heightens the mystery of the pieces the readers are missing.
For example:
John is acting weird around Suzy, and the audience doesn't know why. The truth, while delivered quicker in a flashback, would show that Suzy kissed John when she was drunk and doesn't remember it. This is faster, but not inventive at all, and it takes the reader completely out of the conflict. We can hint at it instead, with John stuttering, maybe staring at Suzy's lips and turning beet red, until it's finally revealed what truly happened.
I hope that different perspective helped!