[This post will replace the one I wrote more rapidly a few days a go on the same subject.]
1. What you are being asked to do is very difficult. Produce college-level writing in Spanish. Think of what you are asked to do in a course in another department. You would probably be embarrassed to turn in a paper with a grammatical or vocabulary mistake in every other sentence, or with a very low level of intellectual content. Writing in Spanish, you still need to produce college-level work, so you need to remember everything you ever knew about academic writing. If you are already a good writer in English, then you need to transfer those skills to the new language. Principles of organization and rhetoric or not fundamentally different between the two languages. For example, you probably know that you wouldn't start a paragraph with "Also..." in English. So why would you start with "También..." in Spanish? That's a weak transition. If you aren't a good writer in English, you are in trouble, because then you have to learn composition at the same time as you struggle to master a foreign language. Write well-developed paragraphs of about 5-6 sentences each, with topic sentences in each paragraph that all support the main thesis statement of the paper.
Because of your language skills, you might unconsciously dumb down the content of your paper. Your sentences might be too short, your vocabulary limited. Don't let poor language skills make you seem less intelligent than you are. Avoid grandiose or obvious statements. "Lorca is a famous writer." When doing a compare and contrast, don't say "There are many similarities and many differences."
2. Read the instructions. Figure out what the professor wants from the paper. It is likely she doesn't want her own ideas from class parroted back.
3. Do not translate your ideas into Spanish. Before you begin writing for the day, look at some authentic Spanish academic writing to get a feel for what your style should be. Borrow some frequently used phrases like "Sin lugar a dudas" or "Así las cosas." A frequent source of unclear writing is the literal translation of phrases that made sense in English but produce garbled Spanish.
4. What did the Spanish language ever do to you? Why are you mistreating it so? Now is not the time to forget elementary grammar. To say "like this" you just need "así," not "como así." "As much" is not "tan mucho" but "tanto." Don't make verbs reflexive or subjunctive in a random way. Distinguish between parts of speech. "Mágico (magic, adj), "magia" (noun). Use technical terminology correctly. Never use the word "cuento" unless you mean the literary genre of a short-story or short oral tale. Look at the corrections of previous Spanish papers you have written, in this class or previous ones. Chances are you are making the same mistakes over and over again. Don't make grammatical mistakes in the title of the paper.
5. Choose quotes carefully. Don't use the quote the professor analyzed in class to make his main point. When you do quote from a text, use the quote rather than merely quoting it. For example, with a long block-quote that occupies a quarter of a page, you need to point out some relevant details after the quotation, not just have it sit there using up space.
6. The good news is that the professor is likely to be more tolerant of some level of grammatical mistakes than a professor in another department. The professor does not expect perfection. A few errors in the use of the subjunctive will not make an A paper into a B-. Really basic mistakes tend to produce more anger than more subtle ones.