Look in this chapter to find answers to questions on:
Q How do I add a guide to a specific position?
A Under the View menu, choose New Guide. Enter the Position you want (and the Orientation) in the dialog.
Q How do I add a guide exactly in the center of an image?
A Try changing the ruler units (View>Rulers) to percentage, and then drag a guide to 50%, using the ruler to determine the position. (To change the ruler units, Control-click [PC: Right-click] anywhere on the ruler and choose a unit of measurement from the contextual menu.) Or, from the View menu, choose New Guide and enter 50% in the Position ...