Basic 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 9 and survives 30 uses; every crafting attempt spends one use.
Walter: Buy it from Walter's General Shop in Dunbarton. (32,500 gold). No special requirements, just gold.
Effie: Buy tailoring manuals directly from Effie's shop at Qilla Base Camp in Port Qilla (32,500 gold). Standard gold purchase, no special currency needed
Granites: Buy a pattern from Granites's general store in southwest Filia. (32,500 gold). Elf-allied humans get a discount, other races pay full price.
Neid: Defeat Neid, the field boss that lives in Lake Neagh inside Abb Neagh.. Fought in the open field, no special pass needed, but it hits hard and works best with a group.
Materials
Expect about 5 attempts per finished item (23% 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
5× Common Leather — low-rank Weaving skill output
Finishing (once per item)
1× Fine Finishing Thread — Buy directly from any general goods NPC shop.