Flashbacks are something of unknown territory for me. They've turned up on occasion but mostly as isolated incidents. Those are simple to handle: they're like info-dumps. My science fiction stories (there are three of them) involve a lot more flashbacks and uses them for character development and backfilling earlier plot points.
What function does the flashback have?
No scene should have only one function. That applies to flashback scenes too.
Character development: Flashbacks are a chance to show-not-tell the reader about important aspects of a character's personality.
Info-dumping: Chunks of world-building can be worked into flashbacks, of course. The entire scene can serve to explain how things came to be in a particular situation, in your story.
When does the reader need to know this?
Connected to "current" events: Flashbacks are like info-dumps in that they always need to be relevant to the story. The best advice I have on when to info-dump is "just after the reader absolutely needed to know this." So the same goes for flashbacks.
Taking a break: If your story has been running hard and fast for a while, you can let it coast a bit while you flashback to something relevant but slower paced. Since flashbacks are in the story's past, they tend to reduce the tension -- the reader already has some sense of what might have happened and you're just filling in the particulars. (That's not to say you can't pack in some surprises, of course.)
When in doubt...
...keep writing, because once you reach the end of the story everything will be much clearer. And you can always fix it in revisions.