(Adjust for your own version of Word)
Open your Word document and look for the References tab.
Put your cursor at the end of the sentence you want to cite (or at the end of the information).
Click the "Insert Endnote" in the style ribbon.
Make sure the endnote and superscript number ended up where you wanted.
If you need to change your endnotes from Roman numerals, click the little pull-down menu.
Use the drop down menu to change your numbers, then click apply.
Sometimes, a source is so awesome, you want to refer to it multiple times in your document. ACS follows this rule:
"Numerical reference citations are numbered consecutively from the beginning of a paper. When, occasionally, a reference is repeated in the text, the original number is used, a new number is not given."
For example, the number 23 can come after 42 in your in-text citation. The easiest way to do subsequent citaitons is to manually add the endnote as a subscript number.
But at the end of the paper, the citations are all in numerical order in the references page.