Almost every word processor allows you to use either footnotes or endnotes. But, if you are typing your essay on an electric or manual typewriter, you will find endnotes to be the preferred method for the simple fact that determining in advance the amount of space the footnotes will take up at the bottom of every page is hard work. There's nothing wrong with endnotes. In fact, they make for an easier read because your reader's eyes are not constantly shifting between the text and the bottom of the page. Of course, flipping between pages of text and pages of endnotes is tiring as well.
Consistency is everything.
The footnote number, in superscript, should be placed at the end of the sentence or quotation. There will come a time, however, when you need to insert two notes within the same sentence. This is perfectly acceptable practice. It all depends on the nature of the thing being footnoted. It goes without saying that your footnotes must be numbered consecutively from beginning to end.