The best advice is to read great authors who've included flashbacks in their short stories. One that comes to mind is Bradbury's, "The Swan," which has a few that are masterfully done. I would say the most common way to insert one is after a page break, to let the reader know it's an entirely different scene, setting, and even time period. You can clue them in with the first sentence, or you don't really have to, as long as there is something there that lets them know it's back or forward in the timeline. You don't always have to do the pagebreaks, and you'll see in Bradbury's short he uses that technique, and also seamlessly transitions into a flashback without having a pagebreak. This is done by a masterful first, and last sentence, which brings the reader out of the current scene without detaching them from the narrative, while also bringing them back in the same fashion. From what I've read, the best flashbacks used no typographical tricks, such as italics, to cue the reader a flashback is taking place. They've let their words do the work for them, and if there was any trick employed, it was nothing but a simple page break.