Mongo's Long Skirt is crafted with Tailoring in Mabinogi (clothes). 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 D and survives 30 uses; every crafting attempt spends one use.
Walter: Buy it from Walter's General Shop in Dunbarton. (1,600 gold). No special requirements, just gold.
Effie: Buy tailoring manuals directly from Effie's shop at Qilla Base Camp in Port Qilla (1,600 gold). Standard gold purchase, no special currency needed
Materials
Expect about 1 attempt per finished item (100% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
1× Cheap Fabric — Buy directly from any general goods NPC shop.
1× Cheap Silk — Mobilizing the Expeditionary Force shadow mission reward
Finishing (once per item)
1× Cheap Finishing Thread — Buy directly from any general goods NPC shop.