Using Document Grids
Photoshop CS6 Tutorial
How to Use Grids in Photoshop
Open a new 500 pixel by 500 pixel Photoshop document (File > New).
To see the Grid, choose View > Show > Grid (Cmd/ ' (apostrophe) (PC - Ctrl/ ' )).
Notice that the grid has a series of dark lines with four lighter division lines between the dark lines.
To change the Grid configuration we'll need to open the Preferences panel (Photoshop > Preferences > Guides, Grid and Slices). For PC users, the preferences are in the Edit menu.
Change the Gridline Every value to 100 pixels and Subdivisions to 10. The Grid updates to reflect your changes.
Select the Rounded Rectangle Shape tool. Set the Tool options to Shape Layer and choose a Fill color.
Click-drag in the document to add your shape. It should snap to the Grid as the shape is drawn. If the Snap feature isn't working, make sure it is activated in the View menu.