Middle-Class 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 5 and survives 30 uses; every crafting attempt spends one use.
Walter: Buy it from Walter's General Shop in Dunbarton. (86,400 gold). No special requirements, just gold.
Granites: Buy a pattern from Granites's general store in southwest Filia. (86,400 gold). Elf-allied humans get a discount, other races pay full price.
Effie: Buy tailoring manuals directly from Effie's shop at Qilla Base Camp in Port Qilla (86,400 gold). Standard gold purchase, no special currency needed
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
3× Finest Fabric — weaving on a loom, higher rank
2× Fine Silk — weaving on a loom, low-mid rank
3× Fine Leather — fragmentation of Fine Leather Strap (or weaving byproduct)
Finishing (once per item)
1× Finest Finishing Thread — Buy directly from any general goods NPC shop.