What's the easiest way to complete this? Is there a really good guide which tells you where to get the item/s required to make each weapon?

first you need the schematics.... they tell you the required items

Get broken steel to raise your level cap. Then there is a perk called warmonger at level 26 that gives insta-schematics thus the only barrier is to retrieve all items the schematics call for. Easiest way if you don't want to find all the locations.