Ella's Vest Skirt 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 A and survives 30 uses; every crafting attempt spends one use.
Red Succubus: Enter Rabbie Basic alone so she spawns as the solo boss and defeat her for a chance at a dress pattern.. She only appears when a single player reaches the boss door in Rabbie Basic.
Pirate Skeleton (Hook): Defeat Pirate Skeletons carrying hooks inside Rundal Dungeon.. No special pass needed beyond normal dungeon entry.
Sources that no longer exist: Production Gachapon.
Materials
Expect about 5 attempts per finished item (20% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
2× Fine Fabric — weaving on a loom, low rank
1× Cheap Silk — Mobilizing the Expeditionary Force shadow mission reward
1× Finest Leather Strap — weaving rank 3-1 on a spinning wheel
Finishing (once per item)
2× Common Finishing Thread — Buy directly from any general goods NPC shop.