Harvard referencing is the most common referencing style in academia and is the style suggested in the EPQ specification.
There are free online reference generators, for example: http://www.citethisforme.com/uk/referencing-generator/harvard
However if you want to be sure and do it yourself the Haravard referencing style is as follow.
Last name, first initial. (Year). Title. Edition (if not the first edition of the book). City of publication: Publisher.
Hawking, S. (2005). A Briefer History of Time. 2nd ed. London: Transworld Publishers.
Last name, First initial. (Year). Article Title. Journal name, Volume (Issue), Page/s.
Jones, D. (2011). String Theory as a Mode for Time Travel. Reviews of Modern Physics, Volume 3 (1), pp. 12-14
Author/Source if no specific author (Year). Title of web document/page. [online]. (Last updated: if this information is available). Available at: URL [Accessed date: Day/Month/Year].
Nerd Herd Corner (2019). How to build a time machine. [online]. Available at: https://nerdherdcorner.com/2019/01/10/how-to-build-a-time-machine/ [Accessed 7 Feb. 2019].