How do you forge weapons out of the boss souls in dark souls

Take the soul of a boss, plus a regular weapon similar to what you want to make, to the giant blacksmith in Anor Londo.

Ring the two bells, go through Sen's Fortress, and then you'll be at Anor Londo. Continue through here until you're in a giant room that has two giants and one archer on a staircase. On the side of the room with the archer, near the top, is an opening you can take to go directly to the blacksmith.