Select the Infuse perk bubble on the weapon you want to boost. A list of possible equipment to sacrifice will appear if you have any in your inventory. Hold square to dismantle said equipment after highlighting it on the list (it will say "Dismantle to Infuse" or something like that). Done. Requirements (from what I can tell): Sacrificed equipment must be in the same equip slot. Sacrificed equipment must have a higher Attack/Defense rating. Sacrificed equipment must be Rare or higher.