Sculpt the form you want to create out of clay. You won't be firing the clay, so there is no need to worry about air bubbles within it--simply shape it with your hands or clay sculpting tools until you have the form you want. For very large objects, try creating a wood frame out of scrap from the theater's scene shop. Cover it with chicken wire to form the basic shape, then cover the chicken wire with plastic sheeting, such as a tarp, and apply the clay over this form.
Dampen the clay as you work with it by wiping it with water occasionally. When you are finished sculpting, cover it with a thin plastic sheet, such as a lightweight drop cloth. Press the plastic into every detail of the clay, making sure it adheres to every detail that you want to show up in the prop.
Shred a large number of brown paper bags into pieces no bigger than approximately 2 by 3 inches, but don't cut them--you want the edges uneven. You'll need to cover the entire object three times over, so shred plenty of paper.
Mix equal parts water and white school glue in a bucket, then sprinkle in a small amount of flour and stir. You only need about a handful of flour per two gallons of glue and water mixture.
Crumple up the shredded paper and dip it in the glue. Squeeze out the excess but make sure the entire surfaces of both sides of the paper are coated.
Apply the paper to your plastic, and continue adding overlapping pieces until you've built up three smooth layers on the entire surface. Try not to build up more than three layers or you may have difficulty removing the paper mache from the clay.
Remove the paper mache from the clay when it is thoroughly dry. You may need to cut or tear the prop in some places to get it off the model, but that's OK. Remove the plastic and discard the clay or store it for reuse.
Patch any tears in the paper mache with masking tape from the back side, then smooth on more glue mixture-covered paper pieces to patch the rips from the outside. Allow it to dry completely.
Smooth the surface of your paper mache if necessary by mixing joint compound, available at home improvement stores, with a small amount of white glue and spreading it over the surface of your paper mache. Sand it smooth when it is dry.
Paint your prop as desired, using acrylic paints as a base coat, then applying acrylic or theatrical paints for details.