Popo's 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 F and survives 30 uses; every crafting attempt spends one use.
Nora: Buy it from Nora at the inn in Tir Chonaill. (400 gold). No special requirement, just enough gold.
Pierrick: Buy it from Pierrick at the general store in Taillteann. (400 gold). Prices drop five percent on Wednesdays from the weekly shop discount.
Materials
Expect about 1 attempt per finished item (99.9% 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.
Finishing (once per item)
1× Cheap Finishing Thread — Buy directly from any general goods NPC shop.