Unpack the zip file in the override folder (you'll have to create one if there isn't one already).
If you purchased the game on Steam, the path should look like this:

C:\Program Files (x86)\Steam\steamapps\common\Pillars of Eternity II\PillarsOfEternityII_Data\override

Activate the mod in the mod sectiona under the options menu inside the game. The items are with the shady merchant on
the second floor of The Kraken's Eye, in Port Maje.