Have only one piece of paper in the printer so it doesn't start going on the next page. When the page is printed out, turn it upside down and on the blank side and stick it back in and continue printing. There is this thing you can click on the computer that has double sided printing even with a single sided printer, but do not click that. It prints out a huge black arrow on the paper that says, "Turn this paper over for double sided printing." I was like seriously?!
If it's a good printer, the print driver will have options for printing double sided on a single sided printer. There may be options that depend on how the pages will be bound. These will result in different instructions on how to orient the paper when you return it to the paper tray.
If you have Microsoft Word, it can handle this even with a bad print driver.