Citations should be placed at the end of the sentence that they are referencing, before any punctuation marks. For example:
Incorrect: This is an example of a sentence with a citation. (Citation)
Correct: This is an example of a sentence with a citation.(Citation.)
Citations should also be placed after any punctuation marks that are part of the sentence that they are referencing. For example:
Incorrect: This is an example of a sentence with a citation." (Citation)
Correct: This is an example of a sentence with a citation."(Citation)