In a grammatical situation where you are using sic to indicate that a word was eliminated, you must place the term "sic" in brackets [sic] and italicize the word [sic]. It should immediately follow the error. For example, if someone writes, "I went to there house," you would write, "I went to there [sic] house" because it should say "their" instead of "there."
Sic is also a term used as the abbreviation or slang for the command you may give a dog as in "Sic' 'em, Terror!" In the case where you are using sic as a command to attack, you place an apostrophe at the end of the term, such as sic'. If you are using the past form of the command sic, then the proper form is either sicced or sicked, according to the The Blue Book of Grammar and Punctuation.
Sic, italicized, but not in brackets may also be used as a homonym for the word sick. In this sense, the meaning of the word is sick as in ill. A homonym is a word with the same spelling or sound, but that carries a different meaning. When you use sic instead of sick, this is an inappropriate use of the word sic.