High-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 1 and survives 30 uses; every crafting attempt spends one use.
Walter: Buy it from Walter's General Shop in Dunbarton. (172,000 gold). No special requirements, just gold.
Effie: Buy tailoring manuals directly from Effie's shop at Qilla Base Camp in Port Qilla (172,000 gold). Standard gold purchase, no special currency needed
Granites: Buy a pattern from Granites's general store in southwest Filia. (172,000 gold). Elf-allied humans get a discount, other races pay full price.
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.