Rulers and Guides

Rulers

To turn Rulers off or on, click the View dropdown menu and check or uncheck Rulers. The shortcut is Command+R.
Rulers start off set at 0,0 (in an x, y coordinate system) in the upper left-hand corner. From this point, the ruler increments (typically inches) extend to the right and down. In order to move this axis, click the upper left-hand corner of the ruler and drag it to your preferred location in your project window. From this new location, the increments will extend to the right and left of the center point and up and down. For example, if you project was 6 inches X 6 inches and you moved the axis to the dead center of your project window, the ruler across the top would have 0 as its middle point and would extend 3 inches to the right and left. This would also be true for the ruler running vertically down the left side, with 3 inches counted up and 3 inches down.
This will allow you to accurately measure the width and length of your presentation. This is particularly useful in posters where pictures embedded in your project may need to be a certain size.
Double-clicking in the same left-hand corner will return the ruler to its original position.

If you click the side of the ruler, you can drag a guide (bright blue line) out from the side. Drag the guide from the ruler to a location that will be helpful for accuracy in your additional edits.
At any time, you can move the guides by selecting the Move tool and dragging them to a new location.
To make the guides invisible but still functional, click View > Show > Guides. This will turn off the visual guides, but will keep them in place. The snapping effect will also remain in place so long as it is still turned on. The keyboard shortcut is Command+H .
If at any time you don't want the guides, use the Move tool to drag them back to the rulers, or View > Clear Guides menu.
To turn guide snapping on/off, go to View > Snap.
To avoid accidentally moving your guides, you can lock them by going to View > Lock Guides or Command+Option+; .