You use sic as [sic] or (sic) when something (a name, a fact, whatever . . .) APPEARS to be an error, it's not - at least not YOUR error. It's what the person actually said, even though they were mistaken, you are showing that you are quoting them accurately.
The word is Latin, from "sic erat scriptum", meaning "thus it was written". You drop it in, in square brackets, immediately after the odd word or fact, to tell the reader: yes, I know this looks wrong, but this is exactly what was said or written, and I have copied it faithfully.
Say a politician writes to you and spells "government" as "goverment". If you are quoting the letter, you reproduce the mistake and add [sic] straight after it: "the goverment [sic] has failed". Now the reader knows the error belongs to the politician, not to you or your sub-editor. Without the [sic], a careful sub would "correct" it and you would lose the very point of the quote.
A word of warning. Use sic sparingly, and never use it to sneer. Peppering a quote with [sic] to make an ordinary person look stupid is cheap and unkind, and good editors hate it. Save it for when accuracy genuinely needs protecting: a misspelt name in an official document, a wrong date in a statement, a slip in a letter you are quoting word for word.
Used well, sic is a mark of a careful reporter. It says: I checked, I did not change a thing, and I want you to know it.