You'll unlock the Itemization feature by visiting the Velvet Room on 8/1. This feature will allow you turn a selected persona to an item. Each persona has a fixed type of item that they can turn into. To successfully itemize a persona, you also need to acquire the necessary material. See the list below:
Item Type | Material Required | Acquisition |
Skill Card | Blank Card | Mementos chests |
Melee Weapons | Black Kogatana | Second-hand shop in Yongen-Jaya |
Ranged Weapons | Model Gun | Get a Mothman capsule toy in Akihabara and trade it to the Covetous Man beside the capsule machines |
Accessories | Black Rock | Street vendor in Shibuya Central Street, in the alley beside Big Bang Burger. |
Armor/ Protector | Black Robe | Tanaka's Shady Commodities, Chests in Palaces and Mementos |
Since the list is too much of a work for me to compile, the wonderful people of this Reddit page has completed the list by sharing their findings in a single Google Sheet file. I strongly recommend checking them it out. Kudos to Sima09 for heading the project and to the awesome contributors.