Consider the information being conveyed in the dialogue of a flashback. Flashbacks are often intended to evoke the haze of memory, with details described in generalized terms. When dialogue appears in them, it needs to hold important information about characters or events. If it doesn't , you may not wish to include it.
Devise dialogue that matches the characters as they were then, not as they are now. Information they currently know shouldn't appear in flashback dialogue, and their personalities may differ slightly to reflect their younger selves. For example, a grizzled police detective may flashback to a conversation he had when he was fresh out of the academy, eager to make a difference and unaware of the countless moral compromises he subsequently made in order to do his job. Such distinctions help fulfill the primary purpose of flashbacks.
Link the dialogue to events or incidents in the storyline's present that explain why you placed the flashback where you did. Flashbacks often require a trigger to signal to the reader that the story is moving into the past, and dialogue can be used to facilitate that. For example, perhaps the protagonist hears a curious turn of a phrase that his alcoholic father used during a key moment in the past, or a suspect drops a line of dialogue that someone heard a masked burglar use during a daring crime. The dialogue thus becomes a dramatic link, providing context to the flashback and fitting it neatly within the story's structure.
Write up the dialogue as you normally would, then shift the verb transition to match that of the rest of the flashback. Grammatical transitions in flashbacks can be used to signal to the reader that the action is no longer in the present. So for example, instead of closing a bit of dialogue with "he said," change it to "he had said." The "had" lets the reader know that the dialogue was delivered in the past. Make sure the transition fits with other, non-dialogue grammatical transitions throughout your flashback.