Start off by grabbing a sheet of plywood. Try to cut it short, at least 8 inches wide and 8-10 inches long. Keep in mind the fact that this is the base of your volcano so in case you want to make a bigger volcano, these measurements may vary.
Now, use a plastic bottle (2 litres) and start building the volcano around it.
You will need salt dough, plaster and Papier-Mache in order to form a mountain. Make sure that the bottle is open and cut from the top and you start building the volcano around it. This means that the bottle will be fully covered with the aforementioned material.
You can slack off while making the model from the bottom, but the top must be extremely tight. Try to put as many layers near the mouth of the bottle as you can. This is because when the volcano will eventually erupt, you wouldn't want your model to break off.
Making ridges and channels will add a livelier picture to the model. Start off by making channels on the top and take them down to the base. You will later notice that once the model erupts, the fluids are going to flow through these ridges and channels.
You will have to wait for the model to dry up completely. This will take about 3-4 hours.
Once it has dried off, start painting it with acrylics or spray paints. You can even add plastic aquarium plants to give the volcano a more realistic picture.
Now mix 1 tablespoon of liquid dish washing soap with 1 tablespoon of baking soda. Then add a few drops of red food colouring to the cup as well.
Pour the mixture inside the plastic bottle which is inside your model.
Put the volcano in an open area.
Use a 1/4th cup of white vinegar and add it to the mixture inside the bottle.
Stand back and enjoy the eruption.