Place a colon after the main part of the title to separate the primary title from the subtitle. Add one space after the colon.
Modern Language Association adds one caveat to this rule: If the end of the title already has punctuation, such as a question mark or exclamation point, then don't add a colon. Just add one space after the existing punctuation.
Write the full subtitle. Capitalize the subtitle in normal title case, says Cornell University Library; begin each major word with a capital letter, but don't capitalize small words such as articles, conjunctions and prepositions.
Italicize the subtitle, just like the primary title if the source is a book. If the source is an article, then leave the subtitle in normal text.
Add a period at the end of the subtitle, except when the subtitle already contains a final punctuation mark.
Add a quotation mark after the subtitle's final mark of punctuation if the source is an article.