Literary criticism, in its simplest terms, is "the study, evaluation and interpretation of literature," most frequently presented in essay format. Think about the literary work you are critiquing, approaching the development of your end product as you would for any essay.
Open your criticism with a thesis statement about the short story you are evaluating. Give your opinion(s) about some aspect or aspects of the work that you are going to discuss and a brief preview of the evidence you are going to use to support your opinion(s). You might focus on effective use of symbolism, the appropriateness of the point of view used to tell the story, the impact of word pictures used by the author to involve the reader in the action - whatever you decide to use as your focus.
In the body of your criticism, present the ideas and illustrations of them that support your thesis position. Use of parallel construction in presenting your supporting statements might add to the weight of your presentation. Quotations from the short story to exemplify the points you make will be worthwhile additions.
Conclude your criticism by restating your thesis and summarizing the evidence you have presented in support of that thesis.