Check the original source of your quotation to make sure it has been copied accurately and that the mistake you have spotted is not a transcription error. It is easy to make slips when quoting, and you can save yourself embarrassment by checking that the error is not one you have accidentally introduced.
Check when your source material was first published before using [sic] to note a word that has been misspelled. A spelling that seems wrong in modern eyes may have been perfectly acceptable if your source was written years ago. If so, it would be inappropriate to use [sic], as the original writer did not make an error. For example, Izaak Walton’s 1653 book “The Compleat Angler” does not contain a spelling error in its title, but today, you would spell the word “complete” in a title or article. If you cannot establish whether a spelling may once have been correct, simply give your quotation as is. Since you are quoting, it is already clear that the words are not your own.
Check any factual error you notice in your quotation with a reliable, established source of information, such as a dictionary, a widely used reference book or an official body. Only use [sic] to draw attention to an error if you are sure you can prove that it is actually an error.
Check that the use of [sic] conforms to house style if you are writing for a specific organization. Many organizations have developed style guides to cover details of how documents should be presented. For example, “The Associated Press Stylebook” that journalists use, directs the writer to put a note to the editors where a mistake is found and not to write [sic].
Write the word [sic] directly after the error to which it refers. As it is a complete word and not an abbreviation, [sic] does not need to be followed by a period. As [sic] always follows a word and never starts a sentence, you will never need to capitalize it.
Place the word [sic] in square brackets, so it stands out from the surrounding text. If the original sentence has punctuation, such as a comma directly after the mistaken word, place this punctuation directly after your closing bracket.
Place the word [sic] in italics if the organization for which you are writing requires American Psychological Association formatting. Practice varies on the use of italics, so use them only where they have been requested. Your finished result should look like this example from Purdue University: Practice varies on the use of irtaclics [sic], so use them only where they have been requested.
Use [sic] in square brackets if the error appears within the quote if you are writing in Modern Language Association style. Use (sic) in parentheses if the error appears after the quote. Only use (sic) for directly quoted material and never use it for a paraphrase. Your finished work should resemble these examples from the MLA: According to the Transportation Department, the trains where [sic] running late that day; Shaw admitted, “Nothing can extinguish my interest in Shakespear” (sic).