Drag to the side the upward-pointing carat-thingamajig that's on the bottom
of the ruler. Hold down the CTRL key as you do so if you want fine control
over the placement.
I believe that "doohickey" is the technical term. At least that's what I
tell my classes. In the spirt of "a picture is worth a thousand words"
(perhaps, 10,000 vague words like thingamajig and doohickey), you might
want to look at: