Verify which style guide applies to your subject. Different subjects use different systems, so it's important to know you're using the right one. Harvard, Chicago and MLA are all popular style guides, although many others also exist. Once you've identified the style you should follow, familiarize yourself with its rules.
Keep a note of any sources you consult as you research your essay. This includes books, chapters, journal articles, newspapers and websites. You may find it useful to keep separate lists for each type of source. The precise information you need varies according to the style guide you are using, but most will require information about the author, publisher and year of publication. It is better to keep a note of too much information than have to go back and check sources later.
Insert a reminder of the sources you use as you write your essay. It is very easy to become confused if you try to insert the sources afterward. Add the information in the way that makes most sense to you. Using your word processor's footnote function is one option, or you could just add the information in parentheses or highlight it in bold. You don't need to include the full references at this stage, just enough information to allow you to identify the source later. Always include the page numbers for any specific quotes or information.
Review your essay once you've finished writing to correctly place and format each citation. Some systems use footnotes or endnotes including the full reference information, while others use a brief in-text citation that refers to a bibliography at the end of the essay.
Proofread your essay before submitting it. You may find it useful to ask a friend or colleague to check the essay as a fresh pair of eyes may spot any errors.