(1) Read the instructions. Does the professor want something specific? Are there special rules she wants you to follow? If the instructions are minimal, follow the MLA handbook and assume that the professor wants a standard literary analysis.
(2) Read the instructions again after your finish the paper. While writing your paper you may have forgotten them.
(3) Make most of your paragraphs around 5-7 sentences. Each paragraph should develop a single idea. Follow all the composition rule you learned in your English composition class, even though you are writing in Spanish. Your paper should have one central idea or thesis.
(4) Don't repeat the ideas that the professor explained in class. A paper is not a test of whether you were listening in class, but an opportunity to do more with the material. Don't choose as the crowning example of your thesis an idea that the professor analyzed in class for 20 minutes. Those are HIS ideas.
(5) Call books and authors by their proper titles and names. Remember that the name of an author for most purposes is both last names. You say "García Márquez," not "Márquez." "Vargas Llosa," not "Llosa." The exceptions are Lorca and Galdós, who can be called by their maternal surname alone. Don't call a woman writer "el autor," or assume that her book is narrated by a man as the default ("el narrador'). Capitalize books, put chapters or titles of poems or short-stories in quotation marks.
(6) Under penalty of a year's imprisonment or $10,000 fine, never call a novel, a play, or a poem a "cuento."
(7) If you have an extensive block-quote, do something with it. Don't just have it occupy space. Choosing really good quotes from a book you have read and making good use of them is very impressive, because few students think to even try doing it.
(8) Take care of the little stuff. Just because you are in an advanced course does not mean that you can forget first-year grammar. There are things that are never done in Spanish, like using the verb ser with a present participle or estar as a copulative between two nouns. The more basic your mistake, the angrier your professor is going to be. Don't make verbs randomly reflexive when there is no need for a reflexive, or randomly subjunctive in a main clause. If all you do is fail to use the subjunctive, and there are no other errors of grammar or usage, the professor will probably be overjoyed. Then all she has to do is have you learn one thing. Conjugate the damned verbs.
(9) Don't translate. Don't think your thoughts in English and then write them down in English. Before you write, read something in Spanish for half an hour to get your brain thinking directly in Spanish. You can take mental notes of certain idiomatic phrases used in scholarly writing and use them in your own paper. "Así las cosas..." Or "Sin lugar a dudas..." These phrases make your writing seem more native, less like a student translating badly from the English.
(10) Don't depend on outside sources if it's not supposed to be a research paper. If you do consult sources, use them minimally, to support your own ideas, and be scrupulous about citation. Never cite Wikipedia.
(11) Read the instructions.
(12) Spanish style tends to use longer sentences. You don't want your style to be choppy. Like this. Vary sentence length, without writing excessively long sentences either. Since your vocabulary is limited in Spanish, you'll have to be even more careful about using the same words over and over again. "demuestra que..." "demuestra que..." If the same word is used five or six times in a paragraph or page, the effect is monotony.
(13) You are in college. Your work still has to reflect a college-level intellectual level, even though your are writing in Spanish. You need to avoid the effect of "regression," where your ideas seem too simplistic because of your language ability. You are being asked to do something quite difficult: college-level writing in a foreign language.
(14) Ask the professor for help. We assume you know how to write a paper already, so we sometimes don't give detailed instructions about how to avoid all these pitfalls. Some instructors have particular things they want and they may not be adept at communicating these expectations. Clarify if there is anything you don't understand about an assignment.