Cores' Healer Dress is crafted with Tailoring in Mabinogi (clothes). This page lists everything needed to make it, checked against the game as it exists today. The interactive planner lives in the Tailoring Companion.
How to get the sewing pattern
The sewing pattern is rank D and survives 30 uses; every crafting attempt spends one use.
Nora: Buy it from Nora at the inn in Tir Chonaill. (3,000 gold). No special requirement, just enough gold.
Galvin: Buy a pattern from Galvin's shop under the Observatory in Emain Macha. (3,000 gold). None beyond having enough gold.
Sources that no longer exist: Production Gachapon.
Materials
Expect about 2 attempts per finished item (50% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
1× Cheap Fabric — Buy directly from any general goods NPC shop.
1× Cheap Silk — Mobilizing the Expeditionary Force shadow mission reward
Finishing (once per item)
1× Common Finishing Thread — Buy directly from any general goods NPC shop.