Some U.S. documents must be notarized. This procedure is an arcane holdover from medieval times (as near as I can figure) where some third party examines a small subset of the known universe (looks at your document), gazes deep into your soul (aka asks for identification), mutters certain incantations (“Sign here, please”) and scrawls magic marks on a piece of paper that you sign (practices vary by location). Congratulations. Do you feel any different?
But I digress.
If you are outside the United States, the way to get this done is to hie thyself to a convenient U.S. Embassy or Consulate. There you will find a practitioner of the ancient art.
More information? Look at the explanation of notarial and authentication services of U.S. consular officers from the U.S. Department of State.