Posted by UWO's Writing Support Centre, this document offers a good introductory outline of how to write an essay of comparison and contrast.

There are several other guides to writing comparative essays you may also wish to consult:

U of Toronto offers one here: http://www.writing.utoronto.ca/advice/specific-types-of-writing/comparative-essay

The UWO Writing Support Centre has numerous other student writing resources for both undergraduate and graduate students, available at this link: http://www.sdc.uwo.ca/writing/index.html?handouts

A more extensive discussion of composing essays of comparison and contrast (with links for still further reading) is available from the University of North Carolina at Chapel Hill: http://www.unc.edu/depts/wcweb/handouts/comparison_contrast.html 

Short link: http://is.gd/24ZwN8