Brielle's Outfit 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 1 and survives 30 uses; every crafting attempt spends one use.
Erinn Merchants' Guild Souvenir Box: Buy the box from Sayiv at the Port Qilla docks during a Seasonal Commerce season. Costs 500,000 Seasonal Ducat, capped at 30 purchases per week
Materials
Expect about 7 attempts per finished item (15% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.