Crumple newspaper into a conical shape about 2 feet long. Tape the wide end of the cone to the bottom of the plastic cup.
Wrap masking tape around the newspaper cone to keep its shape. Use four pieces of tape at the base, lower center, upper center and tip.
Soak a piece of bandage plaster in warm water. Strain the strip through your finger and thumb to get rid of excess water. Place the plaster strip over the newspaper cone and press it flat.
Apply plaster strips over the entire newspaper cone as well as the bottom half of the plastic cup. Don't put plaster over the rim of the cup. Cover any gaps in the "horn" with plaster strips. Before it starts to dry, bend the newspaper cone to resemble a large, curved horn.
Allow the drinking horn to dry completely, approximately 24 hours.
Glue a strip of aluminum foil just under the rim of the cup. Cap the pointed end of the drinking horn with aluminum foil and glue it in place. Glue the faux gems around the rim of the drinking horn and along the horn as desired.
Hold the horn so the rim is parallel with the ground, then pour water or flavored juice inside. Drink from the horn like a victorious Viking warrior.