Selina Lady Dress 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 6 and survives 30 uses; every crafting attempt spends one use.
Fleta: Buy a pattern from Fleta while she wanders Sen Mag Plains near the crossroads. (95,000 gold). She only appears at set morning, afternoon, and evening windows.
Prairie Dragon Raid: Join the field raid against the Prairie Dragon northeast of Maiz Prairie in Rano.. Spawns only in two daily windows, mid afternoon and early evening, and rewards scale with your Contribution score.
Sources that no longer exist: Production Gachapon (2012).
Materials
Expect about 5 attempts per finished item (20% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.