Fun Fact: Voodoo dolls have nothing to do with Vodou.
Dolls, also known as poppets, are wonderfully handy and very satisfying to use. Like an excellent puff pastry recipe is to French cooking, so the poppet is to magic. It's a versatile but simple concept which is endlessly useful. It can be tweaked to work in an array of situations, for a variety of purposes, and it can be as basic or complex as the user needs it to be.
The paradigms which use dolls have their own conventions and methods-- obviously, I'm not going to sit here and go through them all. Rather, I'm going provide a method of making a poppet (a term which I am fond of only because it sounds like "pop it", as in "pop it's head off").
The materials for making a poppet are very run of the mill. Any half way decent fabric store should have the necessary items. For the fabric, cotton muslin is a great choice. It's extremely inexpensive, easy to work with and readily available. Some people prefer unbleached muslin because it looks more rustic and natural. When I use muslin, I enjoy the blank slate whiteness of bleached muslin; it reminds me of soft underbellies, blind cave dwelling shrimp, fat grubs, and crabs without armor.
Get thread to match the fabric, contrasting embroidery floss, sewing needles, a box of straight pins, and good pair of scissors. Use these items only for poppet construction.
On the muslin, lightly mark an outline (in pencil or tailor's chalk) of the poppet, allowing a quarter of an inch on all sides for the seam.
Most poppets are about 5 or 6 inches. If you know how tall the victim is, you may want to scale the doll to size, 1 foot = 1 inch.
Cut out another piece of muslin and it pin securely to the piece of muslin which has the outline of the poppet. Carefully cut the poppet out of the two pieces of pinned cloth.
If the facial features are to be embroidered, unpin the pieces and do this now. Be sure to create the face on the correct side of the fabric since the poppet will be turned inside out after it is sewn together. Drawing on the face with ink (this may bleed), fabric paint, gluing on a photo, and using a computer printer iron on transfer are some other ways of making a face.