Create a mold of the rock by forming a large piece of chicken wire into the shape you desire. Chicken wire is available at all hardware stores and is easily bent into any shape your imagination can create.
Cover the chicken-wire structure in papier-mache. Dip strips of newspaper into the papier-mache mixture and lay them over the mold. Apply three layers of papier-mache to create a sturdy structure. Let this dry overnight or longer.
Turn the mold upside down and remove the chicken-wire lining. The rock should stand on its own at this point and look generally like you have envisioned, size-wise. At this point, the color and texture will be completely wrong and unrealistic.
Create a fiberglass mixture from a kit available at any hardware store. Some prefer to dip strips of burlap into the fiberglass and spread it over the mold for a textured look. Others just create a thicker mix and spread it across the mold using a spackle brush.
Let the fiberglass mold dry overnight in a well-ventilated area. Purchase a rock-colored spray paint at your local paint or hardware store, and coat the mold to give it the proper color.