Create a transition for the character to evoke a flashback. For example, the character sees a photograph of his mother, which causes him to remember something in his past. The transition needs to make sense and be seamless, otherwise the audience is jerked from the present and into the past.
Write the scene header as you would any normal scene in all capitals. For example, INT. MIAMI HOTEL ROOM -- DAY (2011). Press return twice to add two line spaces and then write the scene description of what the character is doing. Include something that triggers the flashback.
Write FLASHBACK TO:, right-hand justified and double spaced, after the scene description. Add two more line spaces by pressing return twice afterwards. Then write another scene header of where the flashback takes place. For example, after being in the Miami hotel room, the protagonist may recall another hotel room. This would be written, INT. LOS ANGELES HOTEL ROOM -- NIGHT (1990).
Add two line spaces and then write the flashback scene description. Once you are finished with the description, add any necessary dialogue. Write the character's name in all capitals and centered on the page. For example, write MIKE. Under the character's name, write the dialogue in the center of the page.
Write another character's name in all capitals if he or she is replying. Add any scene descriptions. The flashback scene is formatted like any other scene in the screenplay; it is usually only one scene. After the flashback is finished, write BACK TO PRESENT, left-hand justified.
Add scene description of what is happening in the present day. Write character names and dialogue as needed, as in any other scene. Continue writing the screenplay as normal.