May I translate Spanish documents myself or may I need anyone else related to any legal services such as a Notary person?

You are not able to translate on your own but you can get a friend who is also fluent in both language to do the translation and have his/her signature notarized.

No, you cannot translate your own documents. However, a translator doesn't have to be "related to any legal services such as Notary person." It should be a neutral person, not related to you who is fluent in both languages. This person has to perform a translation and fill out a certificate of translation.

It is preferable that you retain a qualified Spanish/ English/Spanish translator to assist you in translating your foreign documents, even if you are fluent in both languages. First, you may not be fluent in both languages as you might think you are. Second, you are an interested party with respect to the applications you are filing with CIS and as such, DHS may not accord much weight to documents you have translated on your own. If there are errors or inconsistencies in the translation or information that is damaging to your case, CIS in unlikely to accept your explanations that those error were innocent and not in fact misrepresentations intentionally made by you.

You can translate it yourself as long as you include the following certification of translator: I certify that I translated the attached document from Spanish to English and that the translation is accurate and true. I further certify that I am fully competent to translate from Spanish to English.

You may do the translation on your own, but you should include the translator's certification that you also can sign.