One way is to start with a trigger, like a character's memory or an object that reminds them of the past. Then, gradually introduce the flashback details without disrupting the present storyline.
You could start by having a character have a sudden intense emotion related to the past. That emotion can be the gateway to the flashback. And during the flashback, keep the writing style consistent with the overall tone of the novel. Make the connection between the past and the present obvious so the reader doesn't get lost.
Well, you can build up some suspense or curiosity first. Make the reader wonder about what happened before. And when you do the transition, use clear cues like a change in tone or a specific phrase to signal it's a flashback. Also, make sure the flashback adds value and isn't just there for the sake of it.