It should be on the face-up side of the paper, near the short edge that faces the printer. You'll generally find the File option in the top-left corner of the screen, and Print is an item in the resulting drop-down menu. Doing this will open the Print window.  If you haven't yet opened the item you wish to print, you'll first need to do so. You can also press ⌘ Command+P (Mac) or Ctrl+P (PC) to open the Print window. This section will allow you to select which pages you wish to print. You may have to click a "Pages" circle to select the Page Range option before continuing. These will dictate which of your document's pages are printed during the first round of printing. For example: if your document has ten pages, you would type either 1, 3, 5, 7, 9 or 2, 4, 6, 8, 10. You can see the name of the currently selected printer beneath the "Printer" heading near the top of the window. To change the currently selected printer, click its name and then select the printer you want from the drop-down menu. This will prompt your document to begin printing only the even- or odd-numbered pages from your print job. This will determine which way you re-insert your paper:   Print and pencil mark face-down - Place the print side face-down with the top of the sheet of paper facing the printer.  Print and pencil mark on opposite sides - Place the print side face-up with the top of the sheet of paper facing the printer. Do so in accordance with the penciled mark. The fastest way to do so is to press ⌘ Command+P (Mac) or Ctrl+P (Windows). If you typed even numbers for your page range last time, for example, you'll type odd numbers this time. As long as your pages are arranged correctly, this should print the un-printed pages on the backs of your currently printed ones.

TL;DR in Vietnamese:
Dùng bút chì làm dấu nhỏ trên đầu trang giấy. Nhấp vào File và chọn Print. Tìm mục "Page Range" (Giới hạn trang). Nhập vào số chẵn hoặc số lẻ. Bạn cần chắc rằng máy tính đang kết nối với máy in. Nhấp vào Print. Tìm dấu bút chì trên mặt đã in. Đặt các trang đã in vào lại máy in. Mở lại cửa sổ Print. Nhập vào phạm vi trang khác. Nhấp vào Print.