To those who've surprised me by admitting that they somehow made it through High School without learning how to cite a footnote, here's one style guide's preferred form. There are others (Chicago, MLA, etc.), use any you like but be consistent. If you use Word, it'll do the formatting for you.
If you didn't consult any sources other than our assigned text(s), just cite them once and create an abbreviation to use parenthetically (like "LH" for Little History) with a page reference. Here’s how you might cite Nigel’s claim, for example, that Berkeley was smarter than Johnson realized.:
 Nigel Warburton, A Little History of Philosophy (Yale University Press, 2011), p.88. (Hereafter cited parenthetically as LH p.#)