Consider the information you want to include in your flashback. A good story is lean and to the point. Superfluous flashbacks and endless narratives on a character's past may be interesting to you but can seem dry and dull to the reader. Figure out if the information is necessary to the story and requires a flashback to illustrate it.
Make sure the flashback moves the plot forward. While it relates information about a character or a scene, it also has to reveal more information to keep the plot moving forward. Decide what information the flashback will tell the reader.
Use a flashback after a strong, clear scene. Your reader should know who the main characters are and have an idea of why the flashback is needed. The flashback should not happen in the first couple of scenes in your story.
Describe to the reader what is going on. Write a few sentences to set up the flashback. Often, the reader gets confused and does not realize a flashback has occurred and is trying to make the past information mesh with the present. Avoid this mistake by describing how a protagonist or other major character is thinking back to a similar situation or scene.
Change the verb tenses. Start the flashback using past perfect tense for the first few verbs and then switch to using simple past tense. End the flashback by reverting back to your original tense and describe an action or scene change to indicate to the reader that the flashback has ended.