You'll also find out how many schematics there are for each gun.
The following chart display the names and ingredents required to create all seven custom weapons. Also, you must make sure that you have all the ingredents, because otherwise you wouldn't be able to make a gun. This usually means visiting a merchant (Moira for example) or a laboratory. In order to begin the process you must locate a workbench. I would recommend having at least two schematics before proceeding with the assembling. You won't have to get to all the schematics for each of the seven weapons, but the more you collect the better the quality of the weapon is going to be. There's a total of 23 weapon schematics for you to find.
You will be allowed to create seven custom weapons throughout the course of the game, however this entire mechanism is more complex than you might have anticipated.