First you need to decide what to do. It's best if you choose something you're interested in, like how to cook something, how to build something, or whatever you like.
Then you need to know who your audience will be, so you're not giving technical words to kindergartners or vice versa.
You also need to know what kind of material you will need. A good idea if you're cooking something would be to have a chart listing all the ingredients, supplies, and directions. You should also have all needed materials, and if possible, the finished product, especially if it requires cooking.
For example, let's say you're making milkshakes. You might want a chart listing the following: ice cream, milk, any flavoring or coloring you might want, and a blender.
For a milkshake, the amount of ingredients depends on your taste, but for other recipes, you would need exact amounts.
In the directions, leave out all unnecessary words and phrases and keep it short and to the point.
For milkshakes you could say: put ice cream, milk, and any flavoring or coloring in blender. Set to liquify and leave until it reaches desired texture.
In the case of a milkshake, you don't really need to have a finished one already made.
Before you go up in front of the audience, you should know what you're saying and doing, and practice or rehearse it to your parents, friends, stuffed animals, people off the street, or whatever you prefer.
For your demonstration, you must first tell the audience what you're demonstrating.
After they know what you're doing, you can start your demonstration showing each step in the proper order and going slowly enough for the audience to hear what you're saying. And finally, be sure to speak and look at the audience so they can clearly hear what you're saying.