Applying a custom skin to an auxiliary weapon requires blueprints and an Auxiliary casting toolkit. Blueprints drop in NM dungeons and can be purchased on the auction house. The toolkit is sold by vendors in London for 16,000 PAX.
- Place Demonic Visual Blueprint 1 + Demonic Visual Blueprint 2 + Demonic Visual Blueprint 3 into the crafting panel. Note that they all have a slightly different icon.
- Place the Aux casting toolkit into the Tool slot.
- Click assemble button to create an Aux Weapon Mould: Demonic.
- Simply place both the Aux Weapon Mould and your auxiliary weapon in the assembly window and clicking assemble.
Thank you Icarus23, Makidai and Memorial for the screenshots!