This means that the flashback is never the first scene. It's not even the second scene following a brief, sketchy, introductory "scene" like the following:
Gary stared out his kitchen window. Cold rain beat on the brown grass and bare trees. It took him back to that other rainy day thirty years ago, the day that had changed Gary's life forever ... The reason this is not an adequate first scene to support a flashback is that it's not really a scene at all. Nothing is described besides the weather. We have no idea who Gary is, so we don't care about his past. Why should we? As far as we're concerned, he doesn't yet have a present.
A far stronger approach is to start your story with a scene in story time. It should be an interesting, vivid scene, which brings its character(s) to life for us. It should contain action pertinent to the story's central concern, whether there is a murder, a family argument or a personal internal crisis. It should also go on long enough to really get us into the story. Then you can use the flashback as your next scene once your reader is hooked.
What if your story contains more than one flashback? In that case, it should either be a long short story novel. Most of what you write should actually occur in story time. If you do need two or more flashbacks, intersperse strong present-story-time scenes among them. Don't go immediately from one flashback into an even earlier one. The reader will likely become either confused or irritated, wondering when you're going to actually get on with your main story.
2) Orient us at the start of the flashback in time and space.
Some transitions to flashbacks are so clumsily written that the reader isn't even sure until halfway through the scene that it is a flashback. Others let us know we've moved back in time, but not how far or to what place. A reader who is expending energy trying to figure out where and when he or she is will not be engaged with your story.
3) Use verb tense conventions to guide your reader in and out of the flashback.
Conventions have evolved about using verb tenses to signal both the start and end of flashbacks. Although most readers don't consciously notice these tense shifts, the shifts register below the level of consciousness to signal "Now we've moved back in time" and "Now we've left the flashback to rejoin story time." Using these conventions is the best way to keep your reader from flashback confusion.
If your story is being told in the past tense, then write the first few verbs of the flashback in the past perfect and the rest in simple past. For example, in the above excerpt, some story-time events may be in the past tense ("habits came back," "he knew," "he scanned.") To signal the start of the flashback, you may put verbs in past perfect ("had done," "had dressed," "had bought," "had come," "hadn't even seen"). After that, the rest of the flashback can be in past tense ("eyes were," "they passed," etc.). The reason for this is that an entire flashback in past perfect would be cumbersome, especially if it's very long.
When you're ready to end the flashback, revert to past perfect for the last few verbs. Then use past tense to resume story time. This is one way to come out of a flashback.
As Eddie hustled him away, he had heard people saying something about heart attacks and strokes. Bystanders had made way for them, apparently feeling sorry that Eddie's little boy had seen some stranger at the moment when a vessel in his brain exploded. Schaeffer felt his pulse begin to settle down now. What if your story is being told in present tense? The convention is even simpler. Put story-time action in present tense and put the entire flashback in past tense. When you're ready to return to story time, simply resume present tense.