Adding a vertical and horizontal ruler into the editor can be done by clicking the menu icon up in the top right corner. Simply click Toggle Rulers to turn rulers on or off.
You can add guidelines to both the vertical and horizontal rulers by clicking on the rule where you want to add the guideline. There is no limit to the number of guidelines that can be added to a page and they can be moved freely along the rulers. These lines also act as a snap-to line, so you can use them as on-the-fly alignment lines. To remove a guideline, click its marker on the ruler and then click the Delete Line button.