You make your insertion [in brackets] when using the MLA format. But it has to fit in with the flow of the sentence. Other handy tips. If you omit anything... use ellipses. If you italicize or bold anything, add in brackets after the quote, [emphasis mine]. If you're quoting more than three lines of poetry or [I think it's] two lines of prose, don't keep the quote in the paragraph but set it down a line and indent it.
<--- English major.