You will first discover U-Invent machines in Arcadia which is where you first start gathering and looting items that can be used to create ammo and other items using these machines. All you have to do is to loot enough items to create something, take it to a U-Invent machine and invent at least one item. The trophy will pop with this happens. If you wanna save some supplies for the Inventing 100 items trophy, save just before you invent something, invent it, then re-load your save after the trophy pops.