This is fairly easy. You need to rank up the Hanged Man confidant to unlock this ability, and to do that, you have to visit his store after his card shows up on the city overview. When it does, visit the airsoft shop, talk to him, and he'll give you a bag. You'll have to raise your guts high enough to approach him about it.
Once you have, it will unlock basic customization. Performing this once is fine, though keep in mind that this basic level may not be enough for to upgrade more advanced weapons. Ovviously you could just save, buy a cheap weapon, upgrade it, and then reload your save to save the money.