Decide what you want to write about. Pick a subject that you know well, as the best stories are those that come from experience. Even if the story is an assignment, try to draw on your own expertise to make it more interesting. You do not have to talk about personal events to make a story personal. If it is a subject you care about it will show in your writing.
Write your story as you would any other. Do not think about the story being translated into Spanish. Write it as you would any other story--read it over for accuracy and make sure it sounds right. Reading aloud is a good way to check that all of the sentences flow. Run it through a spelling correction option on your word processing program to verify there are no mistakes.
Run your story through an online translation program. Google Translate and SDL Free Translation are two that are free. You simply enter your text, using the pull-down menu to indicate that you are translating from English to Spanish. Then, click the "Translate" button. Your entire article will transform to the new language.
Check your work. Run the article through at least two translation programs. Compare the two stories in Spanish to check whether both programs provided the same translation. If there are any discrepancies, use your Spanish-English dictionary to confirm. If you are still unsure, try a third translation program, such as SpanishDict.com, and choose the translation that appears at least twice.