Determine what the segue will be that allows the character to transition back into the past in his flashback scene. This could be an old newspaper clipping, a family photo or an old love letter, for example.
Decide what event or point in time the character will flash back to. This could be a high school prom, a funeral or an occurrence from childhood.
Write a few lines that depict the scene in the current time, describing the object that triggers the flashback.
Write the words "FLASHBACK TO:" in all capital letters, right-aligned on the page.
Write the name of the scene location it is transitioning to in all capital letters on the left-hand side of the screen, for example "CATHEDRAL HIGH SCHOOL PROM - 2003."
Carry on with the screenplay as usual, describing the surroundings and characters and introducing dialogue.
End the flashback by writing "BACK TO PRESENT" in all capital letters, right-aligned on the page.