Bracer Knuckle is crafted with Blacksmithing in Mabinogi (weapon). This page lists everything needed to make it, checked against the game as it exists today. The interactive planner lives in the Blacksmith Companion.
How to get the manual
The manual is rank 3 and survives 30 uses; every crafting attempt spends one use.
Fishing: fish up treasure chests anywhere fishable, best odds off the Fishing Boat.
Beach of Scathach: fished up rarely (Rare tier) while fishing at Scathach Beach on Belvast Island. No requirements; walk into the corrupted beach area on Belvast Island.
Lileas: Jousting Arena Manager in Tara. Trade Jousting Points at her shop for manuals. Spend Jousting Points at Lileas's shop by the Tara jousting arena, earned from matches there.
Lileas's Organization Box: given by Hooliger in Tara for a Joustmaster's Emblem. Trade one Joustmaster's Emblem, won from a jousting match, to Hooliger for the box.
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.
Each attempt uses
3× Iron Plate — refine from Iron Ingot (re-refining)
2× Iron Bar — refine from Iron Ingot via Blacksmithing (Refining)
3× Finest Leather Strap — weaving rank 3-1 on a spinning wheel