It's not complicated to learn how to print on both sides of paper, but I bet very few of us actually do it. There are few times when you need to have an actual blank page on the back of a printout, but doing so will save precious trees so let this be a reminder to you!
If you don't know how to print on both sides, simply go into your print settings. For Macs, click the arrow next to the printer and then select "Layout" and turn "Two-Sided" on. For PCs, it's usually known as "Duplex."
And a reminder: If you forget to print on both sides, you can always reinsert printed pages into the paper tray, right side down and print your new project.