In both the memoir and the film, Cheryl Strayed’s Wild presents a great example of writing flashbacks. In fact, the story is almost completely told through the use of the device.
My clients often ask me how, in a narrative, do you make the transition from real time to flashback time. A good technique is to use a sensory detail to link the two time frames. For example, you can have your character hear a song on the radio (this is used to great effect in Wild), and then flash back to a time in her past that song was playing. In a narrative format, you have even greater opportunities; you can use smells or the way something feels on her skin.
So give it a try with this prompt:
Imagine your character is going about her business (make sure to have her in an active state), when there is a sight, or a sound, taste, smell, or feel, that brings her back to another point in time. Always a good idea, when writing to a prompt, to use the first person, present tense. This will put you in immediate contact with the event. You can always rewrite it later in third person past tense, or any tense you choose.
Happy Writing!
Margaret