How to (and how not to) prepare a comparison essay

Teachers quite commonly ask for comparison essays. The thing is, such a structure can be used in many different courses, such as literature (comparing two or more books, periods or authors), history (comparing two or more specific situations of certain characteristics), science (comparing two different theories), etc.
Preparing a comparison essay is not so difficult. Here we will provide you some tips of what to do, and what mistakes to avoid.