A demonstrative essay is about how to do something. Here are some ideas for demonstrative essay topics:
In a demonstrative essay, the thesis is very basic. "It is hard to tie one's shoes." "Tying shoes is a step-by-step process." Even when there is not a natural order, a statement which provides the structure for the essay is still a thesis. "There are many different tools needed by automotive mechanics. I will first consider the wrenches."
The key is to break the subject down into bite sized pieces. You are probably being asked to write two or three pages in an essay like this or perhaps up to five pages. But look at how detailed you can get if you are asked for a twenty page or fifty page paper on the topic.
"There are many different tools needed by automotive mechanics. I will first consider the wrenches. There are three types of wrenches: crescent, box and adjustable. The adjustable uses a thumbscrew device to change sizes. The screw is an ancient concept dating back to the Greeks. The reputed originator of the screw was...
This is almost getting into the research essay, but the point is you can provide as much extra information in a demonstrative essay as you need.
The body of the demonstrative essay follows the thesis very closely. Cover the ground thoroughly and try to imagine a person who knew nothing about your topic reading the essay. Look for jargon, words which are only used by people who are knowledgeable about your topic. All jargon should be explained or replaced by less technical language.
Since the thesis is often not much more than an organizing structure for the body of the essay, the conclusion is similarly vague. Often times it is nothing more than an explicit statement that the lesson has ended. "And with the final pull of the shoelace, the shoe is tied."
What is the marker looking for?
Clear sentences. If it's a How To essay the sentences should be "Do this... then do that..." If it describes something less linear then it should be clearly organized into different sections. Always make sure your basic punctuation and grammar are correct.