Mac OS 9 Example:

When printing, a screen similar to the following will appear:



Underneath the Printer name is a pull-down menu with the word General on it. Open the pull-down by clicking on the arrows on the right edge of the General pull-down.



You'll see a label called Layout in the pull-down menu. Highlight it with the mouse, and then let go.



At the bottom right of the Layout screen, there is text that says Print on Both Sides with a checkbox next to it. Click the checkbox, and now when you print your document, it will print on both sides of the same sheet of paper.

Mac OS X Example:

When printing, a screen similar to the following will appear:



Next to the Presets: pull-down, you will see the word Standard. This means the printer will print your document single-sided by default. To change this, open the pull-down by clicking on the blue area on the right edge of the Preset pull-down.



You'll see a label called Duplex. Highlight it with the mouse, and then let go. The preset field should now say Duplex. Now when you print your document, it will print on both sides of the same sheet of paper.