Portia's Costume (G15) 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 4 and survives 30 uses; every crafting attempt spends one use.
Sayiv (10 Uses): Talk to Sayiv on the docks of Port Qilla and open her sewing pattern and manual list to buy them directly.. Payment is in Seasonal Ducats, earned through Season Trading.
Materials
Expect about 4 attempts per finished item (25% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
1× Ice Fabric — Rare drop from Snow Troll, Bran, or Grim Reaper in Advanced Theater Missions.
2× Finest Silk — weaving on a loom, higher rank
1× Tough Thread — weaving Thin Thread Balls
Finishing (once per item)
1× Ice Silk — Obtained from a Festia Commemorative Gift Box.
1× Frost Thread — Weave 5 Snow Spider's Webs together using the Weaving skill.
2× Fragrant Finishing Thread — Drops from Fionnait in the Forest of Four Seasons.