Decide on the length and aim of your biography. This will determine the amount of time and resources needed to complete it. You may need to adjust your plans accordingly. A novel-length biography covering a person's life in detail will need more time and research than a short biography covering a musician's work, or a homework assignment whose length will be determined by your teacher.
Decide whether the biography will be "authorized," written with the subject's cooperation or participation, or "unauthorized."
Research everything that is pertinent to your subject and your biography. Look at both primary and secondary sources. Primary sources are diaries, memoirs or letters present or created at the time. Secondary sources are anything that analyzes the primary sources, such as other biographies or books and newspapers. Make sure these sources are trustworthy and have references. If your work is published, you do not want to be accused of factual errors or plagiarism.
Remember that facts such as birth dates and the dates of particular events, are important.
Critically evaluate the material you have collected. If you know the subject well, then you may need to rely less on outside sources.
Determine whether your level of Spanish is good enough to write initially in Spanish, or have your English version translated. This is an important decision, as writing in English and then having your biography translated may be expensive. However, if your written Spanish is poor, you may have to start over.
If you choose to write initially in Spanish, it is essential that you have your work proofread and corrected by a literate Spanish speaker or expert. This is especially important if you plan to publish your biography.
If you do not intend to publish your biography, machine translation or even a Google translate of your English source may be options for those on a budget. Machine translation will produce lots of errors, so you or an expert will need to proofread and revise.
Form an outline. Decide whether you are going to order the biography thematically, chronologically, or a combination of the two. This may depend on pivotal events in a subject's life. Read other biographies for inspiration. You may wish to reorder the biography as you go along. This is all part of the editing process. Make sure your final copy is proofread several times both by yourself and others.
If you choose to have the final version of your biography translated, take the time to find a good translator with experience in similar types of translation, to provide the most natural, fluent translation.
Translations are usually charged by the source word. Rates vary depending on the translator, the deadline and other variables.
You can post your potential biography translation for open bidding on sites such as Proz.com or Translatorscafe.com.