Make a list of your strengths. If you can't think of any, try thinking of a time you succeeded at a task. Ask yourself what strengths you used to make that success happen. Think of as many situations where you succeeded as you can and find the common qualities that allowed you to succeed. List these qualities as your strengths.
Make a list of your weaknesses. Think of a time when you have failed or when a project did not come out as you wished, and examine what personality traits contributed to that failure.
Write the essay in the format requested using your lists as references. Emphasize your strengths, but don't exaggerate them. When writing about your weaknesses, show examples of when a weakness might also be a strength, such as turning in a project late because you are so detail-oriented you didn't want to turn it in before it was perfect (illustrating your desire for excellence). It may also be applicable to provide a short plan for overcoming your weaknesses.