There are 7 custom weapons to make for this achievement. To make them you will first need to find a schematic for the particular weapon, then obtain all the parts for said weapon. Once you have all that, find a workbench (Moira's shop in Megaton) and create the weapon. You have at least two chances for each schematic, some as many as four. Use this to find them all.
As for obtaining the parts, they can all be found in/around the houses of Minefield, bought from Moira in Megaton or the junk merchant in Rivet City or purchased at the Scavenger Shop SW of Tenpenny Tower (red building, visible from the entrance to Warrington Station). Only the Nuka-Coa Quantum, Deathclaw Hand and Radscorpion Poison Gland can not be found in those locations. See "The Nuka-Cola Challenge" for a link to those locations. Deathclaw Hands can be looted off Deathclaws in the Deathclaw Sanctuary. Radscorpion Poison Glands can be looted off pretty much any Radscorpion which are all over the place.
For a list of all the weapons and the items needed, see here. It is the large quoted section with all the spoiler tags.