Stitched Long Robe 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 6 and survives 30 uses; every crafting attempt spends one use.
Granites: Buy a pattern from Granites's general store in southwest Filia. (94,200 gold). Elf-allied humans get a discount, other races pay full price.
Materials
Expect about 7 attempts per finished item (16% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
3× Fine Fabric — weaving on a loom, low rank
3× Fine Silk — weaving on a loom, low-mid rank
3× Tough Thread — weaving Thin Thread Balls
4× Iron Plate — refine from Iron Ingot (re-refining)
Finishing (once per item)
1× Finest Finishing Thread — Buy directly from any general goods NPC shop.