A FLASHBACK is a marker, just like a CUT TO, and so you right justify it to show a new scene (even though I can't format it below correctly in this window). When you are finished with the flashback you put BACK TO PRESENT left justified as you jump back into the previous scene.
The the flashback is a scene within a scene:
INT. SAIGON HOTEL ROOM – DAY (1983)
Kim gets up from the sofa. Crosses the room to the window. Gazes down at people
walking along the street. She stares at a mother and a young girl about her
INT. SAIGON HOSPITAL – DAY (1981)
Kim's mother is in a hospital bed. Kim is holding her hand, squeezing hard.
Mother, mother open your eyes.
Kim drops her mother's lifeless hand. She stares with unbelieving eyes.
A voice calls her name, "Kim! Kim!"
BACK TO PRESENT
Kim turns away from the window. Steve is calling her name.
Kim! Kim! Are you okay?