The flash-back golden rule: Memories are not for reminiscing. They serve to navigate the present!
Use these to move the story forward, not as a way of introducing back story. Show information that directly affects what’s going on in your story right now. Only flash back to moments where action is happening and keep it short. Max 300words but preferably break it up in little pieces and spread it around where it would naturally come up in the character’s mind.
Give the reader the information in chronological order
When a character reacts emotionally to something first and then it is explained through flash-back afterwards, the dramatic effect is far weaker than if the sequence is witnessed in chronological order.
Don’t explain in hindsight
John’s dad showed up in the doorway. John’s face turned white at the sight of the ladle. His father used to beat them.
Do give the reader required info ahead of time so he understands
John’s father used to beat them. His father shows up in the doorway. John’s face turns white at the sight of the ladle.
Don’t break up the action to insert a flashback
Emma walks in on the burglar. Burglar attacks Emma. They struggle. Emma kicks him into the bathroom. Emma goes into a lengthy flash back about the doorknob/ lock. She locks him in. The burglar rips the doorknob off when he tries to open the door.
Do give the required info just before the action
Emma is exiting the bathroom, minding to be careful with the doorknob. Emma giggles to herself as she remembers the time she locked herself in when the knob broke off. She walks in on the burglar. They struggle. She kicks him into the bathroom. She locks it from the outside. The burglar rips the doorknob off when he tries to open the door.