To craft a weapon, everything you need is schematics and the materials that these schematics require. You also need a crafting station. Using specific materials affects the final result - it is best to use the best materials possible, to receive the attack/defense values as close to the top value as possible (it displays in the upper-right corner, after you highlight the schematic). Additionally, you learn the information on the available upgrades for the specific item. Is it worthwhile to craft your own weapons? Definitely, it allows you to make it fit your own needs. If you are not that picky, find interesting legendary weapons - the ones that are hand-crafted rarely match them. Armors? Here, with the appropriate schematic for you, they can far exceed the quality of the legendary ones.