How do I get Vordt's Hammer?

Soul Transposition in Dark Souls 3 is the process of converting a Boss Soul into a Weapon, or Ring. In order to create items you must have the specific Boss Soul, you must have the Transposing Kiln which drops from Curse-Rotted Greatwood, and you must speak with Ludleth of Courland in Firelink Shrine.

Ludlet will convert the souls to items.

Source.

PS: For as far as I know, when a boss soul is consumed to gain souls, you lose the soul, and will be unable to forge it to items. Untill you beat the boss again in NG+ (or new game with an other character ofcourse.)

Now that you have consumed the soul of Vordt, you cannot get the hammer (until NG+).

To transform a boss soul into a weapon/spell/ring, you have to get the Transposing Kiln from the Curse-Rotted Greatwood (Undead Settlement boss) and give it to Ludleth of Courland (the guy who sits on the throne in Firelink). Once given, you can trade your boss souls with him.

Source : http://darksouls3.wiki.fextralife.com/Soul+Transposition