Becky Witch 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 3 and survives 30 uses; every crafting attempt spends one use.
Giant Lion: Defeat the Giant Lion field boss in Courcle.. Spawns only in set daily windows.
Sources that no longer exist: Eweca Orb (2013).
Materials
Expect about 10 attempts per finished item (10% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
3× Finest Fabric — weaving on a loom, higher rank
4× Finest Silk — weaving on a loom, higher rank
3× Tough Thread — weaving Thin Thread Balls
Finishing (once per item)
2× Fine Finishing Thread — Buy directly from any general goods NPC shop.