By superscript numbers, which appear outside the punctuation, if the citation applies to the whole sentence or clause.
Example: Oscillation in the reaction of benzaldehyde with oxygen was reported previously.⁴
By italic numbers in parentheses on the line of text and inside the punctuation.
Example: The mineralization of TCE by a pure culture of a methane-oxidizing organism has been reported (6).
Use only one method of in-text citation and be consistent.
Cite the reference in a logical place.
With numerical reference citations, start with 1 and number consecutively throughout the paper, including references in text and those in tables or figures. If a reference is repeated, DO NOT give it a new number, use the original.
When citing more than one reference at one place citing with one of the numeric systems, list the numbers in ascending order and separate them by commas (no spaces in superscript, spaces in line). If they are part of the consecutive series, use a dash to indicate a range of 3 or more.
Example: ...in the literature⁴˒⁵˒⁷⁻⁹
...in the literature (4, 5, 7-9)
Order all references at the end of the paper in numerical order if citing by number (superscript or italics).
Order all references at the end of the paper if citing by Author-Date in alphabetical order by first author's last name.
Using a hanging indent for any second and subsequent lines in your reference.
When citing an online source, indicate [Online] after title and include URL and access date.
Different formats have slightly different rules for citation. See How do I cite section on the left-hand side.