To craft a weapon, find a Weapon Workbench. Once here, perhaps at the Overseer's C.A.M.P. near the Wayward, you are given an Optional Mission to create a weapon. At the bench, choose to Craft and you will be given a selection of basic recipes to build if you have the ingredients readily available. If not, find a town or settlement nearby and start looting stuff to scrap. All you have to do is make a basic weapon for this trophy to register, as well as the mission to be completed.