Using Document Guides
Photoshop CS6 Tutorial
How to Use Guides in Photoshop
Guides and Grid Lines are essential for document layout. Once they are in place, it simplifies object placement, drawing shapes and making selections.
Open a new 500 pixel by 500 pixel Photoshop document (File > New).
Display the Rulers by choosing View > Rulers.
Drag two Guides from the horizontal ruler, then drag two Guides from the vertical ruler. The Guides should form a box shape.
Guides can be customized in the program's Preferences panel (Photoshop > Preferences > Guides, Grid and Slices). For PC users, the preferences are in the Edit menu. You can also open the Guide Preferences by double-clicking a Guide with the Move tool (v key).
Select the Rectangular Marquee Selection tool and click-drag within the Guide box. If the Snap feature isn't working, make sure it is activated in the View menu.
To fill your selection with the Foreground color, press Option/Delete (PC - Alt/Backspace).
Moving the Guides
Once the Guides are added, you can readjust them with the Move tool (v key). To Lock the guide and prevent any movement, choose View > Lock Guides.
To clear all the Guides in the workspace, choose View > Clear Guides.
Guides at a Specific Location
To add a Guide at a specific position in your document, choose View > New Guide.