Consult your printer manual. Some printers can handle 2-sided printing automatically. With a simpler printer, you can print the odd pages and then put the printed pages in the paper supply tray and print the even pages on the back side. Start with a small document. Most people will put the paper in the wrong way on the first try. Get a 4-page document right and then you can print a larger document.
If a page falls out of the page, you will still be able to read it on the back side. This method allows you to easily print a larger document without removing the pages until you get the right size. If you print several documents at once in a single set, it is important that you print each document one at a time. This is because, each page will be larger than the next one when you are finished.