Kirsten Leather Armor is crafted with Tailoring in Mabinogi (armor). 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 7 and survives 30 uses; every crafting attempt spends one use.
Iria Exploration Chest: Use L-Rod Exploration or Land Maker across Iria to uncover hidden treasure chests.. Requires an L-Rod or Land Maker tool equipped to search.
Fishing Boat: Fish from the moored boat south of Rano and open the treasure chests that appear.. Requires the Aspiring Sailor title equipped to ask Myrddin for boat access.
Materials
Expect about 5 attempts per finished item (21% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
2× Finest Fabric — weaving on a loom, higher rank
2× Finest Silk — weaving on a loom, higher rank
5× Fine Leather — fragmentation of Fine Leather Strap (or weaving byproduct)
Finishing (once per item)
4× Finest Finishing Thread — Buy directly from any general goods NPC shop.