Place a 20-ounce bottle onto one of the sheets of poster board with the mouthpiece facing down. Trace around the bottle with a pencil. Cut a straight line from the center of one of the poster board's sides. Then cut out the circle you have drawn.
Hold each corner of the split in the poster board with your fingers, and then fold one side over the other to create a cone shape. Place a small piece of masking tape onto the edge of the overlapped side to hold the cone together. This is now the body of the volcano.
Cut around the bottom of the cone shape with scissors to give it a secure base. Each part of the base should rest onto the surface you are crafting it on. Cut off only small portions at a time to ensure that the shape of the volcano is not compromised.
Create the mouth of the volcano. Cut the tip of the cone so that it roughly matches the size of the bottle's mouth. The mouth of the volcano should fit tightly over the bottle opening.
Place the volcano over the top of the bottle. Then tape the cone to its mouth with masking tape. This creates the internal structure of the volcano, allowing you to add the liquid components needed to make an eruption.
Turn the volcano over, with the bottle attached, and fill it with crumpled newspaper. This gives the volcano's structure stability when it is completed.
Turn the volcano back over and place it onto the second piece of poster board and then tape them together with masking tape. The volcano is now be attached to its base. The structural design is now complete.
Decorate the volcano with either paint and paintbrushes or colored pencils. The base and body of the volcano should be earth tones, whereas the mouth may be colored in a lava-like design, using reds, yellows and others that resemble the fiery appearance of lava.
Add the ingredients to the bottle to cause the volcanic eruption. Fill the bottle almost all the way with water, add four or five drops of detergent, add 2 tablespoons of baking soda and then add 2 tablespoons vinegar. The chemical reaction caused by these ingredients will cause the mouth of the volcano to erupt with a fizzing lava-like emission.