Snapping to Guides and Grid
Photoshop CS6 Tutorial
How to Use Guides and Grid 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).
Activate the Rulers by choosing View > Rulers (Cmd/r (PC - Ctrl/r)).
Drag two guides from the horizontal ruler, then add two guides from the vertical ruler.
To activate the Snap feature, choose View > Snap To. There should be a checkbox next to Guides and Grid. Also make sure there is a checkmark next to the View > Snap menu item.
Click the Rectangular Marquee Selection tool, then click-drag from the upper left guide intersection to the lower right intersection. The selection snaps to the Guides.
Delete the Guides by choosing View > Clear Guides and remove the selection by choosing Cmd/d (PC - Ctrl/d).
To see the Grid, choose View > Show > Grid (Cmd/ ' (apostrophe) (PC - Ctrl/ ' )).
Choose the Rectangle Shape tool and set the Tool options to Shape Layer, then choose a Fill color.
Click-drag in the document to add your shape. It should snap to the Grid as the shape is drawn.