Flashbacks are fine and used all the time. In film in particular, this is part of "show don't tell" the story, and what that phrase originally meant: Anything you want to say, try to put in action and scene instead of dialogue.
Just this last year I must have seen half a dozen shows that begin with somebody saying (essentially) "This is what happened..." Then go to VO for a setup line or two ("John and I just got back from Melissa's party, and he was drunk...") then we are in flashback of John fumbling with the keys at the door or whatever, we are in the scene.
The flashback ends with VO again, the scene transitions to current time, and the narrator is on-screen: "That's all I remember, I woke up in that bathroom stall at the bar."
How you do the transitions is not really up to you, so keep the direction simple or non-existent. The director will decide whether to take your advice or not. Flashbacks can end in various ways.
Be Careful that your flashback scene only contains what the narrator can know. It does not have to be shot from the POV of the narrator, but if the narrator is Amy you can't show in the flashback when John goes outside to the car without her and hides something in the glove compartment, or show John surreptitiously taking a note from his jacket pocket (insert address and time or whatever), and whispering it to himself alone in the kitchen before he throws it away.
Which are obvious examples. The flashback is a scene, so it can be easy to forget in the writing or revision of it that it should only contain what the person flashing-back actually observed and remembered.
If the flashback is long then it can be helpful to interrupt it for questions from an interrogator, or because the narrator said something and then remembered: "No, wait, he didn't -- He usually throws his jacket on the bed, but he didn't. He made a point of hanging it up, right away." Then rewind or revise the flashback accordingly, or to save time just pick up the flashback with John hanging the jacket in the closet and turning, etc.
Just to remind the audience they ARE watching a flashback and a tale from memory.