It’s a good thing you’re thinking of the seam as a join, not a patch. The past should feel like a shadow that moves with the present, not a separate room. Start with a strong present anchor—a character or moment that readers can latch onto—then let the past seep in through choices, memories, or objects that carry weight. Don’t just throw in flashbacks; let them answer a question the current scene poses. If the old and new feel like they’re speaking in different languages, the story will wobble. Keep the voice consistent; the past can be colored by nostalgia, but the present must stay crisp. Remember, blending is a negotiation: the old gives texture, the new gives purpose. If one side dominates, the other is just decoration. Find the point where the grain of oak meets the fresh bark, and that’s where your narrative will feel solid.