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Avoid the use of brackets in editorial copy.  If that is not possible, use brackets to insert page references or editorial explanations within quoted matter.  In other text, parentheses enclose the same types of insertions.

Punctuate bracketed matter as though it were within parentheses. Matter in brackets within a sentence does not begin with a capital letter or take a period, even if it is a complete sentence in itself. It may be followed by a mark of interrogation or exclamation:
            "We walked [there were no horses] across the island."
            "We walked across the island. [There were no horses.] Then we came ..."
            "The key to these twin riddles [why did they carve the statues? why did they stop?] still lies buried here."

When matter within brackets is inserted where a comma belongs, the comma follows the second bracket:
            "After the ball ended [they went home at five], the islanders ...'' ''In a model [below], Alvin explores ..."