Click View and select Print Layout. Your footnote will appear at the cursor, so place the cursor at the end of the text that you want to create a footnote reference for. Click the "Document Elements" tab, and then click the "Footnote" button in the "Citations" section. A footnote will be inserted at your cursor and you will be taken to the footnote text section to enter the contents of the footnote. The footnote text will be at the bottom of the same page as the footnote, separated by a line.  Alternatively, you can press Command + Option + F to create a footnote, or Command + Option + E to create an endnote. If you'd rather have symbols instead of numbers, want the footnotes to appear below the text instead of at the bottom of the page, or want the numbering to start at a different number, you can change this from the "Footnote and Endnote" window. Click Insert and select Footnote.   Click Symbol... to choose a symbol from the Symbol menu. You can choose any character from any font, though the "Symbols" font will open by default.      By default, your footnotes will increase in number throughout your whole document. You can change this so that the numbers restart every page or at section breaks in the document. In the "Format" section, use the "Numbering" drop-down menu to select when you want the footnote numbers to restart.      You can apply your formatting changes to just your selected text, the current section, or your entire document.

TL;DR in Vietnamese:
Chuyển sang chế độ xem Print Layout (Bố cục In). Đặt con trỏ chuột tại nơi mà bạn muốn chú thích xuất hiện. Chèn chú thích. Thay đổi thiết lập chú thích.