A mold is something you use when you wish to later make one or more castings or 3D pieces.  Making a life mask or mold is easy, and a common material to use is plaster-impregnated gauze.  This gauze is the same stuff they used on my arm when I was five, and cracked a couple of bones.  You can find rolls of this stuff in art and craft stores, and, perhaps, at medical supply houses as well.  It is very easy to use: add water and apply.  Cut gauze into two-inch to five inch pieces.  Fill an open mouth container (cut down one gallon plastic milk or juice jug) part way up with water.
For a life mask, cover your victim's face with lots of Vaseline, lie down and get comfortable. Have them close their eyes and put a couple of cucumber slices or vaselined pads on the eyelids.  Dip a strip of gauze in water, then skim off any excess.  Apply the gauze onto the face, getting it down smoothly into every crook and nanny.  Continue adding strips, at odd angles and over-lapping slightly, until the entire face is covered.  You don't want to cover the nostrils, but you can run a thin strip down the nose, from the tip to the lip.  Begin again, and add a second, then a third layer, always getting the strips down smoothly, and without air bubbles.  As the plaster sets, it will get warm and begin to pull away from the face.  When this happens, and the surface starts to feel hard, carefully remove it from the face.  Set your work aside to continue drying.  It will take at least overnight to cure, and fans will help the process.
The next day, use paper towels to remove any globs of Vaseline, then paper towels and rubbing alcohol to remove the rest of the residue.  If you're not concerned with extreme detail, before applying the gauze, cover the greased face with a single layer paper toweling.  This will prevent the Vaseline from coming into contact with the plaster.  If you find that the mask/mold isn't as strong as you'd like it to be, add several more layers of gauze strips to the rough side.  Once the plaster is completely cured, you can prepare the life mask, either as finished work or as a mold for casing 3D faces.  As a presentation piece, seal the inside of the mask with several coats of artistes gesso, paint with acrylics and seal with urethane.  If you wish your work to serve as a mold, paint the inside with several coats of shellac.  Not lacquer, not urethane, but shellac.  Line the mold with a thin sheet of food wrap plastic.  Pour in the  plaster of paris, press in paper clay, or lay in strip paper mache (the choice is yours).  After at least several hours, when the casting material has set up, remove the casting by lifting up on the plastic film.  Set your work aside to cure overnight.
To make a body or torso mold, the process is exactly the same, but you won't need cucumber slices.  An even easier, lighter (and cheaper) molding method is to take the impression with heavy-duty aluminum foil.  “Paint” both sides of the foil form with four coats of undiluted Elmer’s or other PVA glue, allowing each coat to dry before applying the next.  This will make the form stiff and hard, and able to hold its shape.  Trim as necessary, the cover with paper mache.
TIP:  If you just can't find anyone who likes you well enough to submit to this torture, you can use a styrofoam wig head.  The face of the head can be altered with modeling clay to resemble the creature you want.  Also by using an altered wig head, you can cover the face with the plastic wrap and use the papier mache instead of purchasing the plaster treated gauze.  The styrofoam head will provide you access to the entire head and you can slit it down the back to remove it from the head.
This method does not just apply to human or styrofoam heads.  You can also do this method with skulls, gargoyles, etc.  I would recommend using aluminum foil for skulls and gargoyles so that you can form it to the shape of the item you wish to make a mold of.