A critical essay seeks to analyze a work based on many factors, including the author's history and similar works; the intent is to be critical rather than biased, creating an objective summary and examination.
For the short story "How I Met My Husband," the first step is to read and understand the story itself. After you understand the story, try to think of similar works, and read the study guides here on eNotes.com. Read about Alice Munro, the author, and see how her life and experiences informed the story.
Next, think of a solid thesis statement that encompasses a critical view of the story. Remember that "critical" is not "negative," so don't explicitly look for negative traits in the story. Instead, look at the story as if you were considering it for its own subjective quality; you have no stake in the matter, so you can be entirely objective in your criticism. A good thesis for this story might be "The first-person narrative allows intimacy with Edie's story" or "Edie settled for 'good enough' by marrying the mailman." In each case, the thesis should be supported by quotes from the story.
Next, remember to follow the basic outline of an essay: Introduction (with thesis statement), body (with supporting quotes), conclusion (with repetition of thesis if needed), bibliography (with any outside sources). See the Online Writing Lab at Purdue University for all your formatting questions.
Finally, remember that your most important task is to be critical of the story -- again, not necessarily in a negative way. Criticism is all about highlighting what works and what doesn't. Constructive criticism and Destructive criticism work towards different ends; generally speaking, you should not strive for Destructive criticism unless (a) you feel strongly about your position, and (b) you have substantial evidence to back it up.