For this costume, you need a long a-line black or dark grey skirt, a red camisole, a sheer black or dark grey half sleeve top or jacket, a black underbust corset, fingerless lace gloves, short boots, and of course a meat cleaver and rolling pin. You might have a few of these things already. If not, you can likely find them online, second hand, and in a Halloween store (the props). For the hair and makeup, check out the video tutorial below.
The wonderful Grav3yardgirl (Bunny) created this Mrs. Lovett Hair and Makeup video tutorial.