We're not talking about marching for the environment or anything like that here, we're talking about literally demonstrating a concept to a group of people. In most cases what you're basically doing is making a pie chart out of people.
Figure out your strategy. You'll need to group people by some fact you find through research. For example, if you're demonstrating that 1/3 of trash thrown out is actually recyclable, split your group up into 2 groups - a group with 1/3 of the people (the recyclable group), and 2/3 of the people in another group (the non-recyclable group). If you're doing it for water usage, you could take the percentages of water that certain appliances use and put that percent of your total group in the group for the appliance. So if your dishwasher uses 27% (this is not an actual fact, just made up) of the water used in your house, put 27% of your total group in the dishwasher group.
Decide how to get a certain number of people that you want to be there. You'll need an exact number in most cases, so talk to your principal/boss if this sounds like a possibility for something you could do there.
Pick a date and place. You may also want to consult your principal/boss about this if you're doing it at school or work.
Prepare. Know exactly what and how you're going to do before the big day. Also make sure you know some additional information in case anyone has any questions.